Glucose induces the biosynthesis of proinsulin and the conversion of proinsulin to insulin in the isolated pancreatic islets from the rat. When the glucose concentration in the incubation medium is below 75 mg/dl, the proinsulin biosynthesis proceeds at a low, basal level; above 75 mg/dl, it is greatly accelerated. Although the conversion of proinsulin to insulin is also accelerated as the glucose concentration is increased above 75 mg/dl, the increase is much less than that of proinsulin synthesis. We suggest that (1) there is a threshold for the biosynthesis of proinsulin and insulin at the glucose concentration approximately 75 mg/dl, and (2) the process of conversion of proinsulin to insulin seems to be the rate limiting step in the biosynthesis of insulin. (Endocrinology91: 309, 1972)
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Lin et al. (1972) studied this question.