Five ribonucleoprotein (RNP) fractions were isolated from the postmitochondrial supernatant of the pancreas of the guinea pig. Two were obtained from the microsomes which, by deoxycholate (DOC) treatment, were subdivided into a DOC-soluble and a DOC-insoluble fraction. The latter was taken to represent attached RNP particles. Two other fractions obtained from the microsomal supernatant supposedly represent free RNP particles existing as such in the cytoplasm, while a third fraction resisted sedimentation for 20 hours at 105,000 g and is considered to be a soluble nucleoprotein. These fractions exhibited different RNA/protein ratios and also different RNA turnover patterns, as determined after in vivo labelling with adenine-8-C14. However, little discernible differences could be detected in the nucleotide composition of the RNA moieties of these RNP fractions.

Amino acid-"activating" enzymes were found to occur in the fraction containing the soluble nucleoproteins.

The discussion focuses on the relationships between these fractions and protein synthesis in the pancreas, using data given in this and a previous paper, and data contained in the literature.

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