1. Aspects of lipid metabolism, including absorption and depositional processes, appear quite different in fish as compared to homeothermic vertebrates. 2. Dietary lipids in fish are absorbed as fatty acids and as triacylglycerols aggregated into chylomicra particles. 3. Interorgan transport of lipids, like that of mammals, consists of an exogenous (dietary) loop and an endogenous loop. 4. Fish store lipids among several depot organs, including mesenteric membranes, liver and muscle. 5. Several fast-acting and slow-acting agents modulate depot lipid mobilization. 6. Mobilized lipids may be transported in the serum as free fatty acids bound to specific carrier proteins.

Lipid dynamics in fish: aspects of absorption, transportation, deposition and mobilization.

Plasma lipoproteins in fish.

Summary
1. The major characteristic of aquatic Crustacea is ammonotelism as shown by the relative importance of various nitrogenous end-products of their nitrogen metabolism.

2. In contrast to urico- and ureo-genesis pathways, ammoniogenesis pathways have received recent attention; some specific enzymes such as glutamate dehydrogenase, AMP-deaminase and glutaminase are now admitted to play a possible key role in ammoniogenesis of various crustacean species.

3. Processes involved in ammonia output through the gill epithelium (diffusion and/or ionic exchanges) are discussed though few data related to Crustacea are as yet available compared to those obtained in ammonotelic fishes.

4. The effects of some environmental factors as well as the physiological state of animals on nitrogen excretion of Crustacea are envisaged. The effects of temperature, salinity and NH4 concentration in the external medium are discussed first, followed by the changes in nitrogen excretion associated with the moult cycle, the nutritional state of animals and possible neuroendocrine control. It is demonstrated that the response of the excretion rate to these factors presents various patterns according to the species, its osmoregulatory abilities and its body reserves. Changes can also occur in the requisite metabolic pathways, thus increasing the difficulty of generalization.

5. In spite of the great diversity encountered within Crustacea an attempt to bring out general trends of their nitrogen excretion is proposed.

6. The present review is focused on metabolic and physiological aspects of crustacean nitrogen excretion but the significance of nitrogen release in nutrient regeneration in marine and fresh-water ecosystems is foreseen.

Nitrogen excretion in marine and fresh-water crustacea

Elasmobranchs are of metabolic interest for several reasons, including their primitive evolutionary position, their osmotic strategy and their low incidence of neoplasia. Some aspects of the metabolism of elasmobranch fishes are unique when compared with those of the other vertebrates. Although many features of their metabolism can be attributed to their primitive evolutionary position (e.g., fewer isoforms of enzymes and other proteins), some unique features appear to be related to the unusual solute system (urea and methylamines) used by elasmobranchs. The solute system exerts widespread effects, which has an impact on the metabolism of lipids, ketone bodies and amino acids and the structure of proteins and membranes. Effects of urea on the transport of lipid may influence aspects of lipid metabolism, reducing extrahepatic lipid catabolism via effects on nonesterified fatty acid transport and enhancing a need for reliance on ketone bodies. Amino acid metabolism of elasmobranchs is also heavily influenced by the need for continuous synthesis of urea with glutamine as the nitrogen donor. These effects, in turn, may play a role in their low incidence of cancer. Specifically, the reduced availability of glutamine (an important nutrient for rapidly growing cells) coupled with the low levels of nonesterified fatty acids in the blood reduces the availability of molecules essential for tumor growth. This metabolic design may thus provide marine elasmobranchs with a “systemic” resistance to cancer.

Jaws: The Inside Story. The Metabolism of Elasmobranch Fishes

Hearts from 16 species of fishes, including representatives of cyclostomes, elasmobranchs, and teleosts, were examined. As a biochemical index of maximal ATP demand, ATPase activity of crude homogenates of ventricular tissue was measured under conditions which optimize activity of myofibrils and inhibit nonphysiological mitochondrial F₁-ATPase by inclusion of oligomycin in the assay medium. The activities of nine additional enzymes from pathways of energy metabolism and concentration of myoglobin-an intracellular protein involved in storage and diffusion of oxygen-were measured. Of all possible pairwise regressions between these biochemical markers, only the relationships among ATPase, carnitine palmitoyltransferase (CPT), and hexokinase (HK) and between HK and phosphofructokinase (PFK) were significant. The inability of elasmobranch myocardium to catabolize fatty acid is confirmed. ATP yields calculated from HK and CPT and corrected to reflect physiological conditions of the tissue suggest that catabolis...

Biochemical Correlations of Power Development and Metabolic Fuel Preferenda in Fish Hearts

1. 1. A technique is described to study the binding of fatty acids to albumin in serum by use of thin-layer gel filtration. 2. 2. The presence of a protein which binds [14C]oleate is established in higher vertebrates, teleosts and a petromyzone. 3. 3. No fatty acid binding protein was observed in the six elasmobranchs studied and the results with crustaceans were ambiguous. 4. 4. The aspects of functional evolution of serum albumin are discussed.

Fatty acid binding proteins in the serum of various animals

1. 1. The levels of non-esterified fatty acids (NEFA) in the plasma of a variety of animals have been estimated. 2. 2. Only one of seven elasmobranchs contained detectable levels of NEFA. 3. 3. The two crustaceans examined contained very low levels. 4. 4. All the other animals contained circulating levels of a variety of NEFA ranging from 14 to 24 carbon atoms. 5. 5. The elasmobranchs are unique in that they also do not possess proteins in the serum which bind fatty acids.

A survey of the non-esterified fatty acids and binding proteins in the plasmas of selected animals

Sea lampreys,Mordacia mordax, were collected in spring from the Yarra River, Victoria, during their upstream spawning migration, to study the lipid composition in their tissues and plasma and their lipid transport system. Plasma lipoproteins were isolated by preparative ultracentrifugation and their chemical compositions were analyzed. The major classes of lipoproteins were found to be similar to those of man and higher animals. Lipids from the muscle and liver were analyzed for fatty acids. The striking feature of the lipids in the migrating lamprey is the presence of very high levels of cholesterol in both plasma and muscle. The possible metabolic roles of cholesterol have been discussed.

A Study of the Plasma Lipoproteins and the Tissue Lipids of the Migrating Lamprey, Mordacia mordax

A comparative study of the binding of L-tryptophan and bilirubin by plasma proteins.

1. 1. The pathway leading to the excretion of ammonia in the crustacean, Cherax destructor , has been investigated. 2. 2. Injection of ammonium chloride and of serine into the haemolymph always led to a rapid increase in the excretion of ammonia. This was not the case with other amino acids. Isolated gills incubated with serine and threonine but not glutamate and glutamine produced ammonia. 3. 3. Serine dehydratase was present in the gills, midgut gland and tail muscle and serine (0.1 mM) was present in the haemolymph. 4. 4. The hypothesis is put forward that serine may be the ultimate source of ammonia and that this deaminating reaction is not restricted to the gill. Such a hypothesis sees the formation and deamination of serine—from 3-phosphoglycerate to pyruvate (lactate) as a modified glycolytic pathway.

Florence C.I. Fellows

Papers

Fatty acid binding proteins in the serum of various animals

A survey of the non-esterified fatty acids and binding proteins in the plasmas of selected animals

A Study of the Plasma Lipoproteins and the Tissue Lipids of the Migrating Lamprey, Mordacia mordax

A comparative study of the binding of L-tryptophan and bilirubin by plasma proteins.

Nitrogen metabolism and excretion in the freshwater crayfish Cherax destructor