Alex J. Anderson

Characterisation of cellulase activity in the digestive system of the redclaw crayfish (Cherax quadricarinatus)

Abstract Endogenous cellulase activity was identified in the gastric fluid and digestive gland of the redclaw crayfish. Cellulase showed maximal activity from pH 4 to 5 and was stable for up to 2 h at 40°C. Cellulase activity in the digestive gland was unaffected by antibiotic treatment. Taken together these findings suggest a significant endogenous component for redclaw cellulase activity. Partial purification of cellulase activity was performed using anion exchange and gel filtration chromatography. One major and one minor band of activity were identified subsequently by SDS-PAGE and zymography. The molecular weight of the major band was estimated at 40 kDa while the minor band was estimated at 30 kDa. Redclaw cellulase enzymes demonstrated broad substrate specificity, hydrolysing polysaccharides containing β-1,4 and mixed β-1,4 and β-1,3 glycosidic bonds but showed a preference for soluble substrates. Hydrolysis products of cellodextrins of various lengths also showed that the enzymes liberated free glucose. Exposure of redclaw to antibiotics resulted in a dramatic decline in bacterial populations in the gastric contents (>90%) but only a 40% decline in cellulase activity.

Effect of dietary cellulose on digestion in the prawn Macrobrachium rosenbergii

The effects of dietary cellulose on digestion and absorption in Macrobrachium rosenbergii were studied by inclusion of α-cellulose at the expense of starch into four purified diets. The results showed that increasing α-cellulose reduced the apparent digestibility of both dry matter and protein, but also showed that cellulose itself is digestible, with an apparent digestibility of 80%. Increasing dietary levels of cellulose caused an increase in cellulase levels in the gastric fluid, and a decrease in amylase levels in gastric fluid and hepatopancreas. The activity of cellulase was highest between 60 and 180 min after feeding. Increased levels of dietary cellulose were associated with lowered levels of protein in the gastric fluid, and increased levels of protein in the hepatopancreas. Total reducing sugars in the hepatopancreas and glucose in the hemolymph also increased with higher levels of dietary cellulose. Increasing dietary cellulose caused no increase in volatile fatty acids (VFA) in either the gastric fluid or the hemolymph, or in total lipids in the hepatopancreas. It was concluded that the digestion of cellulose in M. rosenbergii was not due to fermentation by endosymbionts producing volatile fatty acids, but to the presence of cellulase which released glucose that was apparently absorbed into the hemolymph, and therefore, dietary cellulose can be a source of both digestible and metabolizable energy for this organism.

Characterization of an insulin-like growth factor (IGF) receptor and the insulin-like effects of IGF-1 in the bony fish, Lates calcarifer

The present work is part of a broad phylogenetic study of the insulin superfamily of peptides in lower vertebrates. In the bony fish barramundi (Lates calcarifer), the presence of IGF receptors were investigated in the liver by means of competitive binding studies. The results suggested the presence of a type 1-like but no type 2-like IGF receptor. We also demonstrated insulin-like effects of intraperitoneally injected recombinant human (rh)-IGF-1 in barramundi with rh-IGF-1 and rh-insulin showing similar effects with respect to induction of hypoglycemia and stimulation of incorporation of [14C]-glucose into muscle glycogen.