J.R. Stark

The role of exogenous enzymes in digestion in cultured turbot larvae (Scophthalmus maximus L.)

Abstract Nine enzymes have been measured in turbot larvae and in the most common live diets (rotifers, Artemia sp. and copepods). At first feeding, turbot was shown to possess the complete range of these digestive enzymes. The activities were also present in the specimens of live diets. The lowest enzymic levels, expressed as units per individual, were found in the rotifers. In Artemia the enzymic contents depended on the nutritional state and developmental stage. Adult copepods showed very high enzymic levels. The significance of these results with regard to the importance of exogenous enzymes in turbot larval digestion is discussed.

Digestion of protein in different marine species

Abstract 1. 1. The digestion proteases in five marine species (Atlantic halibut, Hippoglossus hippoglossus (L); Dover sole, Solea solea (L); turbot, Scophthalmus maximus , (L); European lobster, Hommarus gammaarus (L); and the giant prawn, Penaeus monodon ) have been compared by biochemical methods. 2. 2. The pH profiles for the hydrolysis of casein by extracts from the digestive systems of each species showed different characteristics; extracts from adult halibut, turbot and sole exhibited strong pepsin-like activity; whereas this enzyme was absent in P. monodon and in sole larvae. 3. 3. Although lobster extracts, from either the hepatopancreas or the stomach, showed peaks at pH values of 5.8 and 2.5, this latter activity did not hydrolyse a specific substrate for pepsin. 4. 4. Halibut and turbot digestive extracts contained an activity optimal at pH values in the region of 5.0 resembling a cathepsin-like enzyme; an activity which was not evident in the other species under similar experimental conditions. 5. 5. Although all species possessed trypsin-like activity, the pH profiles of activity in the neutral to alkaline region were unique to each species. 6. 6. The significance of these results is considered with respect to the anatomical differences in the alimentary systems of these species.

Protein digestion in early turbot larvae, Scophthalmus maximus (L.)

Abstract The ability of early turbot larvae ( Scophthalmus maximus ) to digest protein has been measured and compared with that of 30-day-old juveniles and adults. Similar proteolytic levels have been found in larvae collected just before first feeding and in the guts of adult turbot after the lumen contents have been removed. However, strong differences in the enzymic patterns and in the specific activity in (rotifer) fed and in unfed specimens of larvae were detected in 6-day larvae and in 30-day-old juveniles. The results obtained indicated that higher levels of neutral and alkaline proteases were produced as a response to ingesting certain live feeds. A possible interpretation of these results is that, because the elevation of protease levels in the gut of turbot induced by live feeds is a slow process, it may involve the synthesis of new enzyme protein.

Metabolism in marine flatfish. II: Protein digestion in dover sole (Solea solea L.)

The digestive tracts of adult and juvenile Dover sole were examined for protease activities. A pepsin-like protease with an optimal pH value of 1.7 predominated in the stomach region, but the main endoprotease action in the foregut, midgut and hindgut regions was optimal in the range of pH 9.5-10.5 and showed good activity towards elastin orcein. Experiments using synthetic substrates suggested the presence of chymotrypsin- and trypsin-like activities optimal between pH 7 and 8. Collagenase activity was also shown to exist in this pH region. The presence of enzymes corresponding to carboxypeptidases a and b and leucine aminopeptidase was indicated. The possible significance of these results to the farming of Dover sole is discussed.

Carbohydrate digestion in Pecten maximus

Abstract 1. 1. A survey of the carbohydrates present in the digestive tract of Pecten maximus (L.) indicated that the highest enzyme activity occurs in the digestive diverticula. 2. 2. A wide range of di- and tri-saccharides were hydrolysed including cellobiose, sucrose, lactose, maltose, maltotriose and raffinose. 3. 3. The polysaccharides glycogen, starch and laminarin were hydrolysed rapidly but alginic acid, inulin, xylan and chitin were unattacked. Cellulose and pectin were hydrolysed slowly. 4. 4. Fractionation of extracts of the digestive diverticula by chromatofocusing indicated the presence of two β-glucosidases, two α-glucosidases and four enzymes capable of hydrolysing laminarin. 5. 5. The starch hydrolysing enzyme was characterised as an α-amylase.

Metabolism in marine flatfish—I. Carbohydrate digestion in Dover sole (Solea solea L.)

Abstract 1. 1. A survey of the carbohydrases in the digestive tract of juvenile and adult Dover sole indicated the presence of strong amylase and maltase activities. 2. 2. The highest levels of these enzymes were present in the foregut region. 3. 3. Chitinase and chitobiase activities were also detected and shown to be optimal at pH 9.5 and 5.0 respectively. 4. 4. Other carbohydrases were either absent from gut homogenates or were only present at very low levels. 5. 5. The relationship between these results and the nutritional requirements of Dover sole is discussed and comparisons made with studies on other fish.

Metabolism in marine flatfish—IV. Carbohydrate and protein digestion in Atlantic halibut (Hippoglossus hippoglossus L.)

Abstract 1. 1. A survey of the digestive enzymes present in Atlantic halibut has been carried out. 2. 2. The endoproteases pepsin, chymotrypsin, trypsin and elastase and the exoproteases leucine aminopeptidase and carboxypeptidases a and b were identified together with activity towards casein at pH 4.5–5.3. 3. 3. The main carbohydrase activities were α-amylase and α-glucosidase. 4. 4. Although the activity towards p -nitrophenyl N -acetylglucosaminide, was strong in all zones of the alimentary tract, there were only trace amounts of chitinase activity. 5. 5. With the exception of chymotrypsin and elastase, all hydrolytic enzymes were most active in extracts of pyloric caeca. 6. 6. A comparison has been made between the intestinal glycosidases of halibut and those of rat.

Metabolism in marine flatfish. VI. Effect of nutritional state on digestion in turbot, Scophthalmus maximus (L.)

Abstract 1. 1. A comparison has been made between the digestive systems in turbot maintained on a standard diet and turbot form the same stock which lacked food intake for 2 months. 2. 2. The alimentary systems did not differ significantly between the two groups either in weight or in dimensions, although the average weight of the livers in the nutritionally deficient group was 25% lower than in the group fed the standard diet. 3. 3. All digestive enzyme levels measured followed the same general pattern but were lower in the nutritionally deficient fish. 4. 4. The enzymes responsible for digestion of the principle nutrients showed optimum activity in different zones of the alimentary canal. 5. 5. The ratio of amylase to protease activity decreased with age of the turbot, possibly as a result of adaptation to dietary changes.

Metabolism in marine flatfish--III. Measurement of elastase activity in the digestive tract of Dover sole (Solea solea L.).

A critical assessment of different methods for measuring elastase activity in crude preparations has been made using whole intestinal homogenates of Dover sole. The use of the natural substrate elastin or its dyed derivatives gave optimal pH values in the alkaline region (pH 9.4-9.8) whereas artificial substrates showed optimal hydrolysis nearer neutrality in the region pH 8.1-8.2. Exoproteases may interfere with certain assay procedures. The properties of Dover sole elastase have been further investigated using chromatographic techniques which indicated that the main elastase activity has a molecular weight of approximately 19,500 and an isoelectric point in the region of pH 5.7.

Leucine aminopeptidase in the digestive tract of Dover sole [Solea solea (L.)]

Abstract Leucine aminopeptidase from the digestive tract of Dover sole has been shown to have maximum activity at pH 8.3 and at 50°C. The enzyme is activated by manganese ions and inhibited by EDTA. The rate of hydrolysis of leucinamide with uncentrifuged homogenate is four times as fast as when clear supernatant extracts are used. Of the total intestinal leucine aminopeptidase, 42% is present in the midgut, 33% in the foregut and the remainder distributed between stomach, hindgut and rectum. In a study of the effect of inhibitors, TPCK was the most effective, indicating that histidine is probably involved at the active centre.