Pancreatic tissue, (guinea pig) homogenized in 0.88 M sucrose, was fractionated by differential centrifugation into a nuclear, zymogen, mitochondrial, microsomal, and final supernatant fraction.

The components of the particulate fractions were identified with well known intracellular structures by electron microscopy.

The fractions were analyzed for protein-N and RNA, and were assayed for RNase and trypsin-activatable proteolytic (TAPase) activity.

The zymogen fraction accounted for 30 to 40 per cent of the total TAPase and RNase activities, and its specific enzymatic activities were 4 to 10 times higher than those of any other cell fraction. The zymogen fraction was cytologically heterogeneous; zymogen granules and mitochondria represented its main components.

More homogeneous zymogen fractions, obtained by successive washing or by separation in a discontinuous density-gradient, had specific activities 2 to 4 times greater than the crude zymogen fractions.

Chymotrypsinogen was isolated by column chromatography from pancreas homogenates and derived cell fractions. The largest amount was recovered in the zymogen fraction.

The final supernatant had properties similar to those of the trypsin inhibitor described by Kunitz and Northrop.

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