Abstract
The effects of dietary protein on the ornithine decarboxylase (ODC) activity in rat liver were investigated, using whole egg protein, casein, gluten, and zein as the protein sources. In short-term feeding (fast-refed), ODC was induced in response to the nutritive value of dietary protein. In long-term feeding (2 weeks), however, ODC was induced by only the zein diet. At the same time, both the ornithine carbamoyltransferase (OCT) and ornithine-oxo-acid aminotransferase (OAT) activities were markedly decreased. OCT and OAT were specifically inhibited by leucine in vitro. ODC was also induced by an amino acid mixture simulating zein (A-l). Moreover, ODC was decreased by the diet (A-2) in which leucine was reduced from A-l, and was increased by the diet in which ornithine was added to A-2. These results suggest that the induction of ODC by feeding the zein diet for 2 weeks is due to the increased concentration of ornithine, arising mainly from the inhibition of OCT and OAT by leucine, which is contained in a large quantity in zein.
