Carbamyl phosphate synthetase and

Production of urea by cestodes suggests the existence of the urea (Krebs-Henseleit) cycle, which is shown in Fig. 6.11. One of the key enzymes, arginase, has been widely reported in cestodes (796, 185-187). However, some of the other enzymes, notably carbamyl phosphate synthetase and ornithine transcarbamyl, are either absent or present in only low amounts (39) and it is doubtful if a complete cycle operates in cestodes. It is likely that the urea excreted by tapeworms comes from the activity of arginase alone. The uric acid produced and excreted by cestodes probably arises from the breakdown of purines (39). 

Citrulline is an endogenous amino acid involved in the urea cycle. Clinically, it can be used as an arginine substitute in the treatment of inborn errors of urea synthesis, including carbamyl phosphate synthetase and ornithine transcarbamylase. It is also a diuretic. 

It must be noted that only few results have been obtained on fresh biopsy specimens most have been from specimens which have been stored in the frozen state for some time or from specimens of liver which have been removed at necropsy at varying unstated periods after death and kept deep frozen at —15°C for various periods of time before analysis. There is some evidence from our results that at least two, carbamyl phosphate synthetase and ornithine transcarbamylase, of the urea cycle enzyme activities fall off on storage at — 15°C for even 1 day, and this decrease continues over longer periods. Thus carbamyl phosphate synthetase activity in fresh mouse liver is in our experience appreciably higher than in liver kept frozen for some days or weeks. This is borne out by a comparison of the enzyme activities found in human liver obtained by biopsy, measured immediately, after storage at —15°C, and finally in liver obtained at necropsy (Fig. 6). Ornithine transcarbamylase activity in a human biopsy specimen of liver is greater when assayed immediately than when it is kept frozen even a short time or... 

The substances assayed in the reaction mixture are citrulline, in determining carbamyl phosphate synthetase and ornithine transcarbamylase, and urea in determining argininosuccinate synthetase, argininosuccinate lyase, and arginase. 

The enzymes requiring the largest amount of liver are carbamyl phosphate synthetase and, even more, argininosuccinate synthetase. It is therefore most desirable to develop more sensitive methods for their assay. Such methods using radioactive substrates can be devised for these enzymes. 

The st example concerns the intracellulEir localization of the reactions of the ornithine cycle. The enzymes catalyzing two of the reactions, carbamyl phosphate synthetase and ornithine transcarbamylase, Eire found in the mitochondrial matrix while the other three enzymes are in the cjrtosol. As a result, the mitochondrial membrane is interposed as a barrier to the operation of the cycle, necessitating two addi-... 

Certain species of tadpoles excrete ammonia during the premetamorphic phase and urea after the onset of metamorphosis. In those animals, two of the urea cycle enzymes are present in small amounts during the premetamorphic phase—carbamyl phosphate synthetase and arginine synthetase. Metamorphosis can be induced by the administration of thyroxine, and... 

The substrate structures and chemical reactions participating in the conversion of citrulline to arginine are given in Fig. 3. The intermediate formed in the conversion of ornithine to citrulline was found by Grisolia and Cohen in 1952. The structure of this compound was shown by chemical synthesis to be carbamylphosphate by Jones, Spector and Lipmann in 1955. Subsequent advances on carbamyl phosphate synthetase and ornithine transcarbamylase in the laboratories of P.P. Cohen, S. Grisolia, P. Reichard, M. E. Jones and M. Marshall have contributed to the enzymology and elucidation of the reaction mechanisms (cf. Fig. 3). The two enzymes are localized within the liver mitochondria, the last three enzymes of the cycle are in the cytosol. 

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