Effect of a low protein diet on in vitro protein synthesis in thymus, spleen, and bone marrow in young adult rats.

Young adult rats (body weight 90 g) were given a low protein (3% casein) diet for 6 days. Under these conditions, thymus and spleen organ wet weights and DNA content decreased as compared with control rats given a high protein (20% casein) diet. The in vitro incorporation of amino acids into protein was followed using leucine as the radioactive precursor. Ribosomes of tissues from the dietary rats were incubated together with soluble enzymes from liver of rats fed a stock diet, and cofactors. Ribosomal capacity for leucine incorporation into protein diminished in thymus by 33% per mg ribosomal RNA, 77% per total ribosomal RNA, and 43% per mg DNA. In spleen the corresponding reductions were 29%, 81%, and 31%; in bone marrow the decrease was 29% per mg ribosomal RNA, 56% per total ribosomal RNA, and 19% per mg DNA. A reduction in physiological activity is thus evident in these tissues soon after protein restriction.