Degradation of Porcine Insulin and Proinsulin by Rat Adipose Tissue

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Abstract
Degradation of 6.95 × 10−10 M I-125 porcine insulin and proinsulin to TCA soluble radioactivity was assessed in intact and homogenized rat fat pieces and isolated fat cells, with and without Kunitz pancreatic trypsin inhibitor (KPTI), 50μg./ml. Fat cells degraded both substances more actively than fat pieces. Homogenization increased degradation rate in fat pieces but had no effect in fat cells, suggesting the most active degrading sites were readily available to substrate in the fat cell preparation. They, therefore, could be either intimately associated with the cell membrane or, alternatively, intracellular with the fat cell of necessity permeable to insulin. Insulin and proinsulin competed for degradation, and KPTI inhibited only proinsulin degradation. These results are consistent with an initial trypsin-like cleavage of proinsulin with subsequent common degradation pathways with insulin.

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