Muscle protein synthesis: regulation of a translational inhibitor

Insulin and branched-chain amino acids are known to stimulate protein synthesis in skeletal muscle. Extracts prepared from rat diaphragms after incubation in balanced salt solution and glucose alone yielded heat- and acid-stable, TCA-precipitable, nondialyzable factor(s) that inhibit protein synthesis when added to rabbit reticulocyte lysates. Polyribosomal profiles of inhibited lysates were consistent with a defect in peptide-chain initiation. Addition of insulin and amino acids to the diaphragm incubation media partially removed the inhibition seen with the muscle extract and was accompanied by an increase in polysomes and decreased subunits. Similarly, extracts prepared from rat hindlimb muscle 48 h after induction of diabetes were much more inhibitory in rabbit reticulocyte lysates than extracts from control rats. Polyribosomal profiles were consistent with defective peptide-chain initiation. Trypsin treatment before assay abolished the inhibitory activity of muscle extracts from diabetic rats. Because translation-inhibiting peptide(s) appear to be under metabolic and/or hormonal control, their possible role in muscle protein homeostasis warrants further study.