Alterations in Serine Metabolism in Rats Fed Liquid Synthetic Diets. ∗

Abstract

The liquid synthetic diet proposed by Greenstein et al, has been found to support good growth rates, reproduction and lactation in rats (1,2). Therefore, this diet has been considered nutritionally complete and the content of essential plus non-essential amino acids considered optimal. This diet has recently been used as a control diet in studies of hepatic serine biosynthesis in rats. Prompt alterations in the concentration of several enzymes important in serine metabolism were noted. This observation suggests that synthetic diets supporting normal or near normal growth rates may nevertheless require significant compensatory changes in enzyme levels to provide required metabolites. The importance of this observation to the study of alterations caused by specific dietary deficiencies is apparent. These studies also demonstrate the importance of dietary amino acids in the regulation of enzyme levels in serine metabolism.

Methods. Male Osborne-Mendel rats (40-80 g) were used in these studies. Liquid synthetic diets, based on the diet of Greenstein et al{2), (Diet A) were prepared by General Biochemicals, Inc. (Diet No. 116). Identical diets were also prepared omitting serine and glycine (Diet B) or serine, glycine and cys-teine (Diet C). Rats were pair fed these liquid diets from Richter feeding tubes. Control rats were fed, ad libitum, either Purina Lab Chow or a solid 25% casein diet(3). The rats were fed the diets for 7 days, killed and enzyme preparations made from liver as described previously(3). Assays for 3-P-glyc-erate dehydrogenase, using 3-P-glycerate as substrate, serine dehydratase and D-glycerate dehydrogenase have been described(3). One unit of cnzqme activity is defined as a change in absorbency at 340 mp of 0.001 per minute. All materials were obtained from prior sources (3).

Results. Weight gain in rats fed chow, 25% casein or diets A, B and C were equivalent for a 7-day period. However, an increase in hepatic 3-P-glycerate dehydrogenase concentration was noted when rats were fed the synthetic liquid diet (A), (Table I).