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A pigeon pancreas microsomal system supplemented with microsomal supernatant fluid or pH 5 enzyme incorporates radioactive amino acids into purified amylase. Several parameters of the incorporation system are presented. Tryptic digestion of the isolated radioactive amylase shows that the amino acids are incorporated into peptide linkage within the protein, as in amylase synthesized in vivo. The radioactive amylase is first found on the attached ribosomes and with time is transferred to the deoxycholate-soluble microsomal subfraction, which corresponds largely to the content of microsomal vesicles. Only about 30% of the released amylase is found as soluble protein in the incubation medium. The system in vitro thus comprises the vectorial mechanism involved in the transfer of newly synthesized secretory protein across the endoplasmic reticulum membrane into the cisternal space.
Redman et al. (1966) studied this question.