The perfusion of isolated rat liver with media containing 74 μU. bovine crystalline insulin per ml. resulted in a rapid loss of immunoassayable insulin accompanied by the appearance of nonimmunoassayable insulin-like activity as measured in vivo by a rat intraperitoneal assay for insulin activity. The material with insulin-like activity could be absorbed and separated from insulin on Dowex-50W × 8 resin (Na+ cycle). Fractionation on Sephadex Q-100 indicated that it has a molecular weight above 40,000 with possible peaks of activity between 40,000 to 60,000 and above 90,000. Small amounts of nonimmunoassayable insulinlike activity were released by perfused livers even when insulin was not added to the perfusion media. When insulin was added to the media, there was a marked increase in the release of nonimmunoassayable insulin-like activity.

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