BIOSYNTHESIS OF INSULIN IN BOVINE FETAL PANCREATIC SLICES: THE INCORPORATION OF TRITIATED LEUCINE INTO A SINGLE-CHAIN PROINSULIN, A DOUrBLE-CHAIN INTERMEDIATE, AND INSULIN IN SUBCELLULAR FRACTIONS* BY A. K. TUNG AND C. C. YIP

Abstract.-This communication reports the biosynthesis of insulin in the bovine fetal pancreatic slices in vitro. Double-chain proinsulin and insulin were found as major components in the mitochondrial-granule fraction of bovine fetal pancreas. Tritiated leucine was incorporated into a single-chain proinsulin, a double-chain proinsulin, and insulin. Subcellular fractionation of the slices incubated with tritiated leucine showed that radioactive single-chain proinsulin was present in the deoxycholate-soluble microsomal fraction, deoxycholateinsoluble microsomal fraction, and mitochondrial-granule fraction. Labeled double-chain proinsulin and insulin were present in the deoxycholate-soluble microsomal and mitochondrial-granule fractions. These results are consistent with the hypothesis that insulin is synthesized as a single-chain polypeptide on the ribosomes, and that intracellular proteolysis in the subcellular membranous organelles and beta-granules converts the single-chain proinsulin to insulin via a double-chain intermediate.