Dihydroorotic acid dehydrogenase activity of human diploid cell strains.

A gene affecting the final two enzymes of uridylic acid biosynthesis does not affect a third, metabolically adjacent enzyme. Similarly, compounds that increase cellular activity for the affected enzymes do not increase activity for the third enzyme. The pyrimidine pathway can be subdivided into groups of concurrently responding enzymes. These groups may be smaller in human cells than they are in microbial cells.