Studies on the Origin of Pyrimidines for Biosynthesis of Neural Rna in the Rat

Uridine was far superior to orotic acid in labelling the RNA in incubated slices of rat brain. On the other hand, uridine and orotic acid were equally effective in labelling the RNA of hepatic or renal slices In rats in vivo, uridine, but not orotic acid, labelled brain RNA, and the cerebellar RNA contained the most label. In contrast, both uridine and orotic acid labelled hepatic RNA. Only when surgical intervention prevented peripheral metabolism of orotic acid, thereby raising its concentration in the plasma, did neural tissue utilize this precursor for limited biosynthesis of RNA. However, among the tissues studied, the preference for uridine over orotic acid for RNA synthesis was unique to neural tissue.