Islet tissue from the goosefish (Lophius piscatorius) was incubated in the presence of leucine-H3. The tissue was then separated into subcellular fractions and the radioactivity determined in total acid alcohol-soluble proteins, insoluble proteins, and insulin. At any time, microsomal protein had a higher activity than secretion granule protein. Pulse-chase experiments further suggest the microsomes as primary sites of protein synthesis. The data are evidence for microsomal synthesis of insulin and for its subsequent transfer into the secretion granules.

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