Krebs-Henseleit Cycle, enzymes

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). 

Deficiency of one of these enzymes causes accumulation of its substrate and also, to a lesser extent, accumulation of the substrates of preceding enzymes in the Krebs-Henseleit cycle. Thus, in argininosuccinic aciduria, not only is the concentration of argininosuccinic acid in urine and C.S.F. markedly elevated, but the concentrations of citrulline and ammonia in the blood are also raised. A raised concentration of ammonia in the blood is characteristic of all four conditions and probably largely... 

Fig. 6.11. The (Krebs-Henseleit) ornithine cycle. Numbers refer to enzymes as follows. (1) Carbamyl phosphate synthetase (E.C.2.7.2.a). (2) Ornithine transcarbamylase (E.C.2.1.3.3). (3) Arginino-succinate synthetase (E.C.6.3.4.5). (4) Arginino-succinate lyase (E.C.4.3.2.1). (5) Arginase (E.C.3.5.3.1). (After Smyth, 1969.)...

This is an enzyme of the Krebs—Henseleit urea cycle which occurs almost exclusively in liver. It is currently considered to be a good specific indicator of hepatic cell damage. In acute carbon tetrachloride poisoning in swine serum levels are elevated 400-fold. In dogs, the elevation is greater (2000-fold). It may be assayed radiochemically [516] or... 

It was suggested, by Krebs and Henseleit more than thirty years ago, that urea is formed (primarily in liver tissue), by a cycle of reactions between the basic amino acids, ornithine, citruUine and arginine. The original scheme, given in Figure 41, is now known to be oversimplified. Other intermediates are involved, and enzymes catalysing the individual steps in the reaction have been purified and partly characterised. The reactions involved are as follows ... 

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