We investigated the effects of the branched‐chain amino acids—valine, leucine and isoleucine—or their keto analogs, the branched‐chain keto acids—α‐ketoisovaleric acid, α‐ketoisocaproic acid and α‐keto‐β‐methylvaleric acid—on protein synthesis and secretion by monolayers of rabbit hepatocytes incubated with [35S] methionine in pulse‐chase and steady‐state experiments. The branched‐chain amino acids (2.0 mM or 1.0 mM), in the presence or absence of insulin (2 × 10−4 IU per dish) and in both types of experiments, reduced the trichloroacetic acid‐precipitable 35S‐protein secreted into the medium. The branched‐chain keto acids (2.0 mM or 1.0 mM) had a stimulatory effect on secreted trichloroacetic acid‐precipitable 35S‐protein which was observed only by the pulse‐chase technique in the presence of insulin. Immunoaffinity chromatography of medium demonstrated a slight inhibition by branched‐chain amino acids and a slight stimulation by branched‐chain keto acids on secretion of 35S‐albumin and no effect of either treatment on secretion of 35S‐fibrinogen. ELISA analysis of total (i.e., 35S‐labeled and unlabeled) secreted albumin revealed an inhibitory effect of the branched‐chain amino acids in both pulse‐chase and steady‐state experiments, and a small stimulatory effect, in steady‐state experiments, of the branched‐chain keto acids; both effects were insulin‐dependent. Total secreted fibrinogen, under steady‐state conditions, was increased by the branched‐chain keto acids in the presence of insulin, while transferrin production was unaffected by any treatment. Intracellular trichloroacetic acid‐precipitable 35S‐protein was increased by the branched‐chain keto acids in the presence of insulin and was reduced by the branched‐chain amino acids in the presence or absence of insulin, suggesting that the effects of these agents may have been related to synthesis and not to effects on secretion.
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