The relation of ammonia concentration to urea synthesis.

We suggest in Fig. 5 that arginase is common to the two cycles. Although the remaining steps of the two cycles are shown separately in this figure, there seems to be no objection to the assumption that the enzymes catalysing the remaining steps are common to the two cycles, provided that in physiological conditions these enzymes, as is the case with arginase, are likely to be far from the point of saturation. Thus provided that the enzymes of the partial systems that have been investigated in the last ten years are capable of transforming simple derivatives ofornithine and citrulline corresponding to our OX, OY, etc., this scheme provides a way of reconciling the whole body ofwork up to date on the mechanisms of urea synthesis.