Download Digestive enzymes of the West African giant land snail, Archachatina

334s Biochemical SocietyTransactions ( 1 99 1 ) 19
Digestive enzymes of the West African giant
land snail, Archachatina marginata.
FEBISOLA E. ABADOM* and DAVID W. MILSOM
Department of Biochemistry and Molecular
Biology, University of Manchester, Oxford
Foad, Manchester, M13 9PT, U.K.
Department
of
Biology,
Lagos
State
University, Apapa, Lagos, Nigeria.
The West African giant land snail is a
potential source of high quality protein,
rich in essential amino acids [l], and
consumed on a small scale in the Third
World, whose need for such food sources is
well known. Its large size, rapid growth and
prolific habit should make this snail an
ideal subject for cultivation, but no large
scale culture schemes have materialised as
yet.
Successful culture would be aided by a
detailed knowledge of the digestive capacity
of the snail. We have investigated the
distribution of the digestive enzymes in the
gut of the snail, together with some of
their properties. We have also attempted the
separation of the enzymes responsible for
carbohydrate digestion.
Four disaccharidases were assayed by the
release of glucose from disaccharides by the
glucose oxidase method and polysaccharidases
were assayed by the increase in reducing
power on hydrolysis of two polysaccharides.
The herbivorous habit of this snail is
reflected in the relatively high activities
of
carbohydrate
digesting
enzymes,
particularly cellulase (l.7mmoles of glucose
equivalents released from carboxymethyl
cellulose per min per 200g snail) with other
activities in descending order of amylase >
cellobiase > lactase > sucrase > maltase.
Analysis of tissue extracts of salivary
gland, crop, stomach, intestine, digestive
gland and the liquid contents of the crop,
the crop juice, revealled that the bulk of
the carbohydrate digesting activity was
concentrated in the crop juice. For the
disaccharidases, more than 90% of the
activity was recovered in the crop juice,
most of the remainder in the digestive gland
and negligible amounts in the other tissues.
Cellulase activity was distributed almost
equally between the crop juice and the
digestive gland. The rather low protease
activity, measured by
the release of
peptides from casein, was found almost
entirely in the digestive gland.
A digestive gland location for these
enzymes suggests an intracellular form of
digestion of these macromolecules, as has
been shown in other molluscs [ 2 1 .
A low lipase activity, measured by the
release of fatty acids from an olive oil
substrate, on the other hand, was found
largely in the crop juice.
Odiete and Akpata [ 3 1 have demonstrated
that the cellular origin of the crop juice
enzymes is the digestive gland. These
workers also concluded that cellulose is
digested largely by enzymes made by the
snail and not by the gut flora since the
microorganisms isolated from the snail’s gut
failed to digest cellulose. We have not
investigated the possible contribution by
microorganisms.
To learn more of the properties of the
carbohydrases we performed some kinetic
experiments on the whole crop juice. All of
the carbohydrases showed pH optima around pH
5.7, very close to the observed pH of the
crop juice itself (pH 5.8). When activity
against substrate concentration was plotted,
amylase, sucrase and cellobiase gave a good
fit with the Michaelis Menten equation,
while maltase showed pronounced substrate
inhibition and lactase activity seemed to
fit the kinetics of a mixture of two
enzymes. Cellulase activity gave a poor fit
with
all
these
curves.
Some
kinetic
parameters are recorded in Table 1.
-.
TABLE 1
-
__
Km values for disaccharidases
in the crop juice of the West
African giant land snail
Km (mM)
cellobiase
Ki (mM)
0.86 A 0.1
sucrase
14.9
maltase
0.4
lactase (1)
123.0
lactase ( 2 )
34.8
A 1.3
_f
0.1
41.0
11.8
A 6.3
_f
9.0
.~
-
Maltose, sucrose and cellobiose are
natural
substrates
for the snail and
activities against them show expected low
Km values, particularly maltose, even though
it has a strong, and unexplained, substrate
inhibition effect. Lactose, on the other
hand, is unlikely to be used by the snail
and high Km values are reasonable. Act,ivity
against this substrate may be the result of
nonspecific R-galactosidase activity.
Preliminary
attempts
to purify
the
enzymes using ion exchange and hydrophobic
column chromatography resulted in partial
separation of three distinct. groups o f
activities.
The
four
disaccharidases
chromatographed together on DEAE cellulose,
but were separated from
amylase and
cellulase. Sucrase and ma1 tase ( a-1 ink
hydrolysers) were less hydrophobic than, and
were separated from cellobiase and lactase
(R-link hydrolysers) on octyl sepharose
columns.
We conclude that a complex system of at
least three enzymes, o r groups o f enzymes,
hydrolysing a-linkeddisaccharides, R-linked
disaccharides and polysaccharides exist in
the crop juice of the West African giant
land snail.
Miss
F.E.
Abadom
is
financial
support
from
Commission.
grateful
for
the
European
1. Mead, A.R. ( 1 9 8 2 ) Malacologia 22, 489493
2. Myers, F.L. & Northcote, D.H. ( 1 9 5 8 )
Biochem. J . 35, 639-648
3 . Odiete, W.O. & Akpata, T.V.I. ( 1 9 8 3 ) J.
Moll. Stud. Suppl. 1 2 A , 122-125