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/. Embryol. exp. Morplu Vol. 31, l,pp. 169-181, 1974
169
Printed in Great Britain
Isozymes of lactate dehydrogenase (LDH) in skeletal
tissues of the embryonic and newly hatched chick
By PATRICIA A. COFFIN AND BRIAN K. HALL 1
From the Department of Biology, Life Sciences Centre,
Dalhousie University
SUMMARY
The time of appearance and the relative activity of isozymes of LDH were studied in portions
of the skeleton of the embryonic and newly hatched chick by starch-gel electrophoresis.
The following tissues were examined for presence or absence of isozymes: mesenchyme
of the hind limb as it separated into chondrogenic and myogenic tissue; the development of
the cartilaginous model of the tibia and its subsequent replacement by bone; the development of the quadratojugal (a membrane bone) with and without secondary cartilage.
LDHX and LDH2 were present in all tissues from the earliest time at which the tissues
could be detected histologically. Thus these isozymes are ubiquitous. They were also the
predominant isozymes during the first and second weeks of development of all the tissues
and continued to predominate in the quadratojugal after hatching.
LDH3, LDH4, LDH5 appeared after the initial differentiation of the tissues, arose at earlier
ages in cartilage than in bone, and became the predominant isozymes earlier in cartilage (14
days of incubation) than in bone (20 days of incubation).
These patterns of isozymes were related to metabolic activity of the developing tissues,
especially to aerobic and anaerobic glycolysis.
INTRODUCTION
In almost all vertebrate tissues examined the enzyme lactate dehydrogenase
(LDH) has been shown to consist of five distinct isozymes (isoenzymes) (Locke,
1963). The total concentration of LDH and the proportions of the five isozymes
relative to one another have been found to vary from tissue to tissue within the
same individual, to vary during development within a given tissue and to reflect
differential gene action. The five isozymes, designated LDH! to LDH 5 , result
from the combination of the products of two separate genes, designated A and
B (Markert & Moller, 1959). LDHX and LDH 5 possess four identical subunits
produced by genes B and A respectively. Isozymes 2-4 represent mixtures of
the products of the two genes as follows:
i
A0B4
1
LDH 2
A^a
LDH 3
A2B2
LDH 4
AgBi
LDH 5
A4B0
Author's address: Department of Biology, Dalhousie University, Halifax, N.S., Canada.
Requests for reprints to B. K. Hall.
170
P. A. COFFIN AND B. K. HALL
Within a given species the isozymes have similar molecular weights (130000135000; Wieland & Pfleider, 1961; Locke, 1963; Wilkinson, 1970; Pesce et al.
1967) and primary, secondary and tertiary structures (Kaplan & Ciotti, 1961).
They differ in their net electric charge (as evidenced by differential migration
in an electrophoretic medium) and in their amino acid composition (LDHX has
more aspartic and glutamic acid and less lysine and arginine than LDH5,
Locke, 1963; Wilkinson, 1970).
LDH is involved in the final steps of glycolysis and glyconeogenesis, mediating
the breakdown of pyruvate to lactate in the absence of oxygen. The isozymes
differ in their catalytic activity, for LOf^ is inhibited by lower concentrations
of pyruvate than is LDH 5 (Kaplan & Goodfriend, 1964; Wilkinson, 1970;
Latner, Siddiqui & Skillen, 1966; Lindy & Rajasalmi, 1966; Stambaugh &
Post, 1966; Latner & Skillen, 1968). Under conditions of low oxygen tension
LDH 5 would be expected to predominate over LDHX.
The fact that the isozymes may be readily separated by electrophoresis, that
their biochemical activities differ, and that not all isozymes appear in a given
tissue at the same time makes them ideal molecular markers for the study of
cellular differentiation and developmental physiology. The pattern of isozyme
activity is tissue rather than species specific, and is related to the physiological
activity and microenvironment of the tissue (Cahn, Kaplan, Levine & Zwilling,
1962; Markert & Ursprung, 1962). For example, LDHi predominates in heart
muscle, in breast muscle of birds capable of long sustained flight, and in avian
embryos, whereas LDH 5 predominates in mammalian skeletal muscle, in breast
muscle of birds which fly in short bursts and in mammalian embryos (Latner &
Skillen, 1968; Markert & Ursprung, 1971). These distributions have been correlated with local oxygen tensions, pyruvate inhibition and lactate accumulation
(discussed by Stambaugh & Post, 1966).
During development and at rest, the oxygen levels in the breast muscles of
the birds mentioned above are likely to be similar. However, the muscles of
the birds which show short, active bursts of activity contain LDH 5 to cope
with sudden oxygen depletion and a build-up of lactate, whereas those engaged
in long flights have high levels of LDHX to ensure against muscle fatigue.
Lindy & Rajasalmi (1966) determined the effects of various oxygen tensions
on the synthesis of isozymes of LDH in the heart, liver and muscle of the chick
embryo. Embryos exposed to high levels of oxygen (40 %) showed high levels
of LDH l5 those incubated in low levels of oxygen (15 %) showed high levels of
LDH 5 . Goodfriend, Sokal & Kaplan (1966) determined that the heart of the
Salk Monkey normally contained little LDH 5 . However, heart cells cultured in
less than 10 % oxygen showed enhanced synthesis of LDH5, whereas LDH 5
was suppressed below normal in oxygen tension of more than 10 %. Cells from
the heart of the embryonic chick (Cahn, 1964) and from the cortex of the
mammalian kidney (Guttler & Clauson, 1969), when cultured in vitro, responded
to variations in oxygen tensions in the same manner.
LDH isozymes in skeleton
111
In the light of the known role of oxygen tension in regulating the activation of
particular isozymes of LDH we felt that a study of an organ consisting of two
tissues whose development and function was favoured by high and low oxygen
tension respectively would be of interest. The skeleton provides such an organ
for the differentiation, development and function of bone requires high levels
of oxygen, whereas cartilage requires only low levels of oxygen (Bassett &
Herrmann, 1961; Bassett, 1964; Hall, 1969, 1970). We studied the development of the tibia in the embryonic chick for the appearance of isozymes of
LDH. Specifically we examined the appearance of the isozymes during the
initial separation of the limb mesenchyme into its chondrogenic and myogenic
portions, during the development of the cartilaginous model, and during the
subsequent replacement of the cartilage by bone. For comparison a membrane
bone, the quadratojugal (QJ) was also studied. The QJ has no primary cartilaginous model but does develop areas of secondary cartilage four days after
the initial deposition of intramembranous bone (Hall, 1968). Starch-gel electrophoresis was used to visualize the isozymes (modified from Shaw & Prasad,
1970).
MATERIALS AND METHODS
Eggs (Gallus domesticus) were incubated in a Humidaire forced-draft incubator at 37-5 ±0-5 °C and 54±2 % r.h. Fifty trials using a total of 600 embryos
were run.
Tissues were taken from embryos of the following ages: quadratojugal (QJ),
9 days of incubation to 6 days post-hatching; whole tibiae, 6-11 days of incubation; separated bone and cartilage from the tibia, 6 days of incubation to 6
days post-hatching; marrow-free bone shafts, 13 days of incubation to 6 days
post-hatching; myogenic tissue from the limb, 6-18 days of incubation. Tissue
from 12 embryos was pooled for each run. This was necessary in order to
obtain sufficiently intense staining of the isozymes from the QJ and younger
samples of tibia.
The tissues were dissected from the embryos, freed of adherent tissues, placed
into a glass tissue grinder, homogenized in 2 vols. of haemolysing solution
(50ml 0-01 M - K 2 H P O 4 , 50ml 001 M - K H 2 P O 4 , 5 mg EDTA, 1 drop ^-mercaptoethanol, pH 7-0), centrifuged at 5500 rev/min for 15 min and either used
immediately or stored for no more than 4 days at 4 °C. Such storage had no
effect on the isozyme pattern.
For preparation of the starch gels 44 g of potato starch (Connaught Laboratories, lot no. 289-1) was dissolved in 400 ml of gel buffer (1-05 g K2 HPO 4 and
0-45 g citric acid/1, pH 6-0), rapidly heated over a bunsen burner to lyse the
starch granules, de-aerated under negative pressure, poured into a Plexiglass
mould and allowed to set for 2-^-2^ h at room temperature followed by 1^-2 h
at 4 °C.
Haemolysate was applied to slots in the gel and the slots sealed with a viscous
172
P. A. COFFIN AND B. K. HALL
mixture of Vaseline-mineral oil. The bridge buffer consisted of 29 g K 2 HPO 4
and 11-4 g citric acid/1 of distilled water (final pH 7-0) and the electrode buffer
of 10 % NaCl. Electrophoresis was carried out horizontally at 4 °C with an
applied current of 35-40 mA at 160 V d.c. for 18-181 h.
At the end of the run the isozymes were visualized by incubating slices of the
gels in the following staining solution at 37 °C for 40-75 min (prepared immediately before use with chemicals from Sigma Chemical Co.);
Nicotinamide adenine dinucleotide (NAD)
Nitro-blue tetrazolium (NBT)
Phenazine methosulphate (PMS)
0-lM-NaCN
0-5 M tris-HCl buffer, pH 7-1
1-0 M-Na DL-lactate, pH 7-0
Distilled water
25 mg
15 mg
2 mg
2-5 ml
7-5 ml
5-0 ml
35-0 ml
The gels were then rinsed in running tap water for 5 min and fixed for 24 h
at 4 °C in 50 ml of the following fixative (glacial acetic acid imethanol: distilled
water; 1:10:10).
RESULTS
All of the tissues studied contained LDHj and LDH 2 from the earliest age at
which differentiation could be detected by routine histological methods.
Mesenchyme within the limb destined to form cartilage could be dissected
free from mesenchyme destined to form muscle as early as 6 days of incubation,
at which age both tissues contained only the isozyme LDHX and LDH 2 . As the
primary cartilaginous model of the tibia developed other isozymes appeared.
By 8 days of incubation LDH 3 was present in detectable amounts; LDH 4
appeared at 9 days and LDH 5 at 15 days (Fig. 1). The appearance of LDR-,
coincided with a decrease in the amount of LDH t and LDH 2 . LDH 5 further
decreased later in development so that by 20 days of incubation, i.e. immediately
prior to hatching, LDHX could no longer be detected. Thus early in the development of the cartilage of the tibia, LDHX and LDH 2 predominated, whereas
later in embryonic life and post-hatching, LDH 3 , 4 , 5 predominated (Fig. 1).
A similar initial pattern was seen in the mesenchyme of the limb destined to
form muscle. From 6 to 8 days of incubation only LDHX and LDH 2 could be
detected. At day 9 LDH 3 first appeared, to be followed on days 10 and 11 by
isozymes LDH 4 and LDH 5 (Fig. 1). The appearance of LDH 5 was not correlated
with a concomitant decrease in LDHj as was the case in the primary cartilage
of the tibia. In contrast to the cartilage of the tibia, LDHi was the prominent
isozyme within the muscle.
A different pattern again was seen in the bone of the developing tibia as it
replaced the primary cartilaginous model (Fig. 2). Differentiating osteoblasts
and osteocytes were not histologically visible in significant numbers within the
LDH isozymes in skeleton
6
8
10
12
14
16
Incubation
18
20
'I
3
173
5
Post-hatching
Days
Fig. 1. A summary of the LDH isozymes found within (a) muscle, (b) epiphyseal
cartilage, (c) endochondral and subperiosteal bone, (d) membrane bone, (e) membrane bone + secondary cartilage. The dashed lines outline the ages at which
particular tissues were studied; the areas within the solid lines indicate the times at
which isozymes LDHX to LDH3 were found; black areas indicate the predominant
isozymes, both during development (6-21 days of incubation) and during the first
6 days post-hatching.
tibia of the embryonic chick until 10 days of incubation. However, as early as
8 days of incubation the pattern of isozymes of the cartilaginous model differed
from that of the diaphyseal bone. By 9 days of incubation isozymes 3 and 4 were
both detectable within the cartilaginous tissue. However, only LDHX and LDH 2
could be detected within marrow-free bone up to 15 days of incubation. LDH 3
was first detected at 15 days, LDH 4 at 16 days, and LDH 5 at 17 days of incubation.
(Fig. 1). The cartilage from the tibia contained all five isozymes by day 15,
with LDHi present in the greatest concentration. L D ^ predominated in the
bone until the time of hatching. That is, during the last third of development
the isozymes characteristic of areas of anerobic metabolism predominated in
embryonic cartilage, whilst the isozymes characteristic of aerobic metabolism
174
P. A. COFFIN AND B. K. HALL
17
(b)
14
18
Fig. 2. Zymograms of LDHX to LDHg from: (a) the QJ (a membrane bone; 9—18
days of incubation), (b) endochondral and sub-periosteal bone from the tibia (8-18
days of incubation), (c) marrow-free bone from the tibia (13-18 days of incubation),
(d) muscle from the hind limb (13-18 days of incubation).
LDH isozymes in skeleton
175
predominated in embryonic bone. After hatching the pattern within the bone
of the tibia changed with LDH 4 and LDH 5 predominating and LDHi disappearing, so that post-hatching both cartilage and bone exhibited similar patterns of
LDH isozymes (Fig. 1).
Surprisingly, the pattern of isozymes found in the membrane bone studied,
the quadratojugal, differed from that found in the endochondral bone, the
tibia. Bone begins to differentiate in the QJ late in the seventh day of incubation,
but because of the very localized nature of the centres of ossification at that age,
QJ's from 9-day embryos were the earliest examined in this study. At 9 days of
incubation the only isozymes found in the QJ were LDHX and LDH 2 , a situation
similar to that observed in the early differentiation of the other tissues studied.
LDH 3 appeared at 11 days but only in very small quantities, and the two other
isozymes characteristic of anerobic metabolism (LDH4 and LDH5) were not
detected at all (Fig. 2). This is obviously in contrast to the endochondral bone
of the tibia, where all five isozymes were found and where LDH 4 and LDH 5
predominated in late embryonic and early post-hatching life (Fig. 1).
The pattern in the QJ+its hook, i.e including the secondary cartilage which
develops late in the tenth day of incubation, was the same as that for QJ bone
alone except for the transitory appearance of LDH 4 between 15 and 19 days of
incubation. (Fig. 1). Thus the QJ possessed a much more aerobic pattern of
isozymes that either the cartilage or the bone from the tibia.
Fig. 1 summarizes the tissues studied and indicates which isozymes predominated at various times during development and during the first 6 days of
post-hatching life. In muscle, primary cartilage, endochondral bone, secondary
cartilage and intramembraneous bone LDHi and LDH 2 predominated during
embryonic development. This situation persisted into adult life in all tissues
except the primary cartilage and endochondral bone of the tibia where a shift
to the more anaerobic isozymes, LDH 3 to LDH 5 occurred.
DISCUSSION
It has been found that there is a gradual transformation of the pattern of
isozymes during the ontogeny of a variety of tissues, and that an abrupt change
to the pattern characteristic of the adult tissues occurs just before, or just after,
hatching or parturition (Masters & Holmes, 1972; Takasu & Hughes, 1969 a, b).
The cartilage and bone of the tibia studied herein conformed to this pattern.
The quadratojugal did not, as will be discussed below.
All the tissues examined in this study possessed significant levels of LDHi and
LDH 2 up to at least the eighteenth day of incubation, and in the QJ these isozymes predominated into adult life. The predominance of LDH! and LDH 2 is
characteristic of avian embryos and contrasts with the characteristic abundance
of LDH 5 in mammalian embryos (Schultz & Ruth, 1968; Markert & Ursprung,
1971). This difference in the two groups of vertebrates is related to the fact that
176
P. A. COFFIN AND B. K. HALL
the oxygen tension in ovo is greater than that in utero and that, unlike placental
mammals, avian embryos have no mechanism for the removal of lactate from
the egg. Thus LDHJ and LDH 2 are ubiquitous isozymes within the tissues of
the embryonic chick and represent a generalized adaptation of the embryonic
tissues to the low oxygen tension prevailing in ovo. Once their tissues are exposed
to the conditions of oxygen availability after birth or hatching the pattern of
isozymes of homologous tissues become similar in both birds and mammals.
That tissue-specific isozyme patterns, common to a wide variety of species,
result from exposure of the same tissues in all species to similar environmental
conditions at maturity is supported by the work of Markert & Masui (1969)
on the penguin. The penguin is a diving bird and Markert & Masui observed
that all isozymes were abundant in all the tissues of the adult, in contrast to
the dominance of particular isozymes described above. The presence of relatively large quantities of all five isozymes was attributed to the influence of
transitory high and low levels of oxygen and the consequent necessity to function under both aerobic and anaerobic conditions. Thus their results further
indicate that the supply of oxygen plays a large role in determining the relative
activity of the A and B genes of LDH and so influences the pattern of isozymes
of LDH present within a particular tissue.
The isozymes of LDH of tibiae from selected mammals have been studied
without separation into cartilaginous and osseous fractions (Semb, 1971).
Semb determined that the predominant isozymes of the tibia of the adult
mammal were LDH 4 and LDH5, as was found to be the case in the tibiae of the
post-hatching chicks studied herein. Tushan, Rodnan, Altman & Robin (1969)
also found that LDH 4 and LDH 5 predominated in mammalian articular cartilage maintained in vitro. However, the pattern of isozymes of cartilage and bone
have not been examined during embryonic development, and although both
tissues contain the same complement of isozymes at maturity (as shown herein)
the pattern of isozymes differs during development.
As early as at 8 days of incubation, before the differentiation of bone cells
could be detected histologically within the tibia, there was a difference in the
isozymes present in the cartilaginous diaphysis when compared with those in the
epiphysis. During the first 2 weeks of embryonic development LDHX and LDH 2
were common to both cartilage and bone, whereas LDH 3 and LDH 4 , the isozymes associated with anaerobic glycolysis, were found only in cartilage. The
early activation of LDH 3 and LDH 4 indicates that adaptation to anaerobic
glycolysis normally occurs earlier in cartilage than it does in bone. This anaerobic pattern also came to predominate in cartilage earlier than it did in bone
(14 versus 20 days of incubation, Fig. 1).
Chondrocytes have been found to develop under conditions of relative avascularity and low oxygen tension both in vivo and in vitro (reviewed by Hall,
1970), to lack the respiratory and oxidative enzymes necessary for aerobic
processes and hence to participate in anaerobic glycolysis (Bywaters, 1936;
LDH isozymes in skeleton
111
Whitehead & Weidman, 1959; McLean & Urist, 1968; Takada, 1966a, b).
Cartilage has a high content of lactic acid (20-40 mg/100 g wet weight) but
only 1 % of the amount of LDH found in the heart or liver (SokolofT, 1969). The
pattern of isozymes within the cartilage of the chick studied herein conformed to
these features. However, Whitehead & Weidman (1959), Balogh, Dudley &
Cohen (1961), Takada (1966a, b), Greenspan & Blackwood (1966), Pawelek,
(1969) and Wilsman & van Sickle (1971) have observed that the hyphertrophic
zone of primary cartilage does possess oxidative enzymes and does function
aerobically. According to Balogh et al. (1961), Walker (1961), Fullmer (1964,
1965), Woessner (1965), Balogh & Hajek (1965), Gibson & Fullmer (1966),
Takada (1966a, b), Chokshi & Ramakrishnan (1967) and Dixit (1969), osteoblasts, osteocytes and osteoclasts possess all the enzymes necessary for participation in active aerobic metabolism. Thus osteogenesis is enhanced in areas
of high oxygen tension where aerobic metabolism can predominate. The
diaphyses of the younger embryos studied by us consisted of both hypertrophic
cartilage and of newly formed bone and the aerobic pattern of isozymes observed
could reflect either the pattern of the chondrocytes or of the bone.
The osteoblasts and osteocytes of the tibiae began to develop significant
levels of the anerobic isozymes by 15 days of incubation. It would appear that
late in development the activity of gene A begins to increase whilst the activity
of gene B decreases, that more of the anerobic isozymes are produced and that
the cells of the bone begin to function anaerobically.
Both cartilage and bone have similar isozyme patterns after 20 days of
incubation, and both have been found to participate in anaerobic glycolysis after
hatching (McLean & Urist, 1968; Bywaters, 1936; Whitehead & Weidman,
1959; Borle, Nichols & Nichols, 1960; Flanagan & Nichols, 1964). Can this
switch be related to a decrease in oxygen levels during the latter stages of
development ?
By 15 days of incubation the bone of the tibia is well vascularized and there
is a considerable amount of bone marrow plasma within the shaft (Romanoff,
1960). One would expect that the presence of the plasma would indicate that
levels of oxygen within the bone would be high, enabling the cells to function
aerobically. Isolated marrow cells and bone cells from the calvariae of the rat
both have high levels of oxygen consumption (7-9 /d/h/mg protein) and lactate
production (6-7-5 //.mol/2 h/mg protein; Smith, Johnson & Severson, 1973).
However, it was found by Semb (1971) that red marrow plasma, as is present
in the tibia of the chick, contains principally LDH 4 and LDH5, participates in
anaerobic glycolysis, and effects the type of isozyme present within the adjacent
bone. The development of isozymes characteristic of anaerobic glycolysis in the
bone shafts of the tibiae of the chick could, in part, be due to the progressive
development of a metabolically active tissue competing for the available
supplies of oxygen. The fact that the marrow, containing extensive blood
supplies and high levels of oxygen, metabolizes anaerobically, suggests that the
12
E MB 31
178
P. A. COFFIN AND B. K. HALL
oxygen levels are not sufficient to meet the requirements of the two metabolically
active tissues. The abrupt decrease in LDHj in the bone shafts at 20 days of
incubation might also be explained by the progressive increase in muscular
activity and the general decrease in available oxygen to the entire embryo at
this time. Thus, although bone is more vascularized than epiphyseal cartilage,
the levels of oxygen available at the two tissues are similarly low toward the
end of the incubation period and at maturity so that gene A is very active in
both tissues and LDH 4 and LDH 5 predominate.
The competition of bone marrow and muscle for oxygen could also explain
why the bone of the QJ shows a more aerobic pattern of isozymes than does the
bone of the tibia, for the QJ is smaller and associated with much less muscle
and marrow than is the tibia. How a simple factor such as oxygen could regulate
the rate of gene transcription or translation remains to be elucidated.
Other factors also effect LDH within the skeleton. The repair of fractured long
bones, arthritis, and the early stages of ectopic bone formation are associated
with enhanced activity of LDH 4 and LDH 5 (Vessel, Osterland, Beam &
Kinkel, 1962; Buring & Semb, 1970; Gudmundson & Semb, 1971; Reddi &
Huggins, 1971) perhaps because of the proposed regulatory role of lactate on
proline hydroxylase and on synthesis of collagen (Slavkin, 1972). Hormones
such as cortisone and parathyroid hormone are also known to decrease overall
levels of LDH within both bone and cartilage (Laskin & Engel, 1960; Herrmann-Erlee, 1963; Deguchi & Mori, 1969; Meyer & Kunin, 1969 a, b). The
action of hormones on the pattern of individual isozymes has not been studied.
The membrane bone of the quadratojugal was characterized by isozymes
associated with aerobic metabolism and did not show a shift towards LDH 4 and
LDH 5 with time. LDHX also predominates in the membrane bone of the rabbit
mandible (Bruce & Strachan, 1967).
Those specimens of the QJ which were run with secondary cartilage present
exhibited LDH 3 from 11 days of incubation onwards. Eleven days of incubation
is the time when the secondary cartilage first appears on the membrane bones of
the embryonic chick (Murray, 1963; Hall, 1968). By 16 days of incubation a
considerable portion of the secondary cartilage has been resorbed. LDH 4
appeared during the fifteenth day of incubation on those samples containing the
secondary cartilage.
Cartilage and bone therefore differ in their isozymes of LDH during development. An anaerobic pattern appears earlier in primary epiphyseal and secondary cartilage than in bone and an aerobic pattern persists for longer, both in
intramembranous and in endochondral bone. This difference can be related to
the fact that chondrocytes develop under conditions of low oxygen levels whereas
osteogenesis requires relatively high levels of oxygen. By 20 days of incubation
the supply of oxygen to the cells of the bone has decreased, due to the general
decrease in oxygen in ovo, the increasing mass of the shaft, and to the competing
influences of active bone marrow and muscle. At maturity both cartilage and
LDH isozymes in skeleton
179
bone within the tibia possess a predominance of LDH 4 and LDH 5 and hence
participate in anaerobic glycolysis. The bone of the QJ initially develops under
similar conditions of oxygen supply as does the tibia and thus has a similar
pattern of isozymes which indicates predominant aerobic glycolysis and which
is retained after hatching.
Financial support from the National Research Council of Canada (grant no. A 5056 to
B.K. H.) is gratefully acknowledged. The experimental work was carried out while P.A.C.
was in receipt of a Dalhousie University Entrance Scholarship. Dr L. E. Haley and Mr P. G.
Meyerhof provided expert advice concerning the electrophoresis.
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SEMB,
(Received 21 May 1973, revised 18 August 1973)