The biogenesis of chromaffin granules in adrenal medulla was investigated with [35S]sulfate, which is known to label the sulfated mucopolysaccharides of these organelles. Retrogradely perfused bovine adrenal glands were pulse-labeled with [35S]sulfate. Four hours after the injection of the isotope the adrenal medullae were subjected to subcellular fractionation. It was found that the soluble material which had incorporated [35S]sulfate was specifically confined to cell particles identified as newly formed chromaffin granules. These granules differ from mature ones by their lower density as determined by centrifugation through sucrose gradients. The labeled material appeared to be associated with the acidic chromogranins, as shown by polyacrylamide gel electrophoresis and isoelectric focusing. In experiments with rats in vivo it was found that newly formed chromaffin granules which had incorporated the isotope took about 48 hr to mature.
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