Factors Affecting Activities of the Urea Cycle Enzymes

Factors Affecting the Activities of the Urea Cycle Enzymes 

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 

Comparison of carbamyl phosphate synthetase (CPS), argininosuccinate synthetase (ASS), argininosuccinate lyase (ASL), and arginase (A) activities in the human liver assayed immediately after biopsy (A), after storage at —15°C (B), and at necropsy (C). 

However, not all the results in glycogen storage disease can be dismissed as resulting from imperfect laboratory technique. Enzyme deficiencies could occur if the mutation was in an operator gene controlling more than one adjacent gene (Jl), and it is not impossible that such a situation exists in the hereditary metabolic errors of urea formation. 

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