Metabolic effect of orotic acid in rat L6 myoblasts.

Orotic acid (OA) is an intermediate in the pyrimidine pathway. The main source of OA in the human and animal diet is bovine milk and its products. OA significantly inhibited the stimulation of protein synthesis by FCS derived growth factors in L6 myoblasts. An increasing OA concentration (0.001 mM, 0.01 mM, and 0.1 mM) in a medium containing 2% FCS decreased the proliferation of L6 myoblasts (as measured by the number of cells) as well as the incorporation of traced thymidine into cellular DNA after 24 h incubation. This mitoinhibitory effect was accompanied by a significant reduction of ornithine decarboxylase (ODC) activity, an enzyme which is believed to be a valuable index of cell proliferation. A drop in the cytosol spermine level of L6 myoblasts treated with OA also occurred. A simultaneous, significant, compensative increase of S-adenosylmethionine decarboxylase (SAMDC) activity was noted. The addition of putrescine (2 microM), a product of ODC activity, abolished the depressional influence of OA on protein synthesis in L6 myoblasts, thus confirming its interference with the polyamine pathway. Dibutyryl-cAMP (0.05-0.2 mM) or adenine (25 microM) supplementation, regardless of OA concentration, significantly attenuated the mitoinhibitory effect and its inhibitory action on protein synthesis. This may suggest the existence of a purine deficiency in OA-treated myoblasts. These experiments indicate that increasing cellular OA concentration inhibits cell growth probably by disturbances in the chain of early events (synthesis of cyclic purine mononucleotides, and activity of the ODC/polyamine system), normally evoked by growth stimulating factors.