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The characteristics of the specific binding of labelled insulin to turtle thyroid microsomes were investigated. Binding experiments were performed in Krebs–Ringer bicarbonate buffer (pH 7·4) at 25 or 4 °C for different periods of time. Dissociation of the labelled insulin from the binding sites was also evaluated. It was found that the binding is dependent on time, temperature and microsomal protein concentration, with an optimum pH of 8·0. Unlabelled insulin and pro-insulin competed with the labelled insulin, binding in direct proportion to their biological activities, while glucagon and growth hormones did not compete for the binding sites. Scatchard plot analysis established the presence of binding sites of high and low affinities, and the rate of dissociation of bound insulin was considerably increased by the addition of unlabelled insulin. Both results are compatible with a negative co-operativity site–site interaction model. Trypsin abolished the insulin binding. These findings indicate that the microsomes from the turtle thyroid gland contain specific binding sites for insulin. However, preincubation of microsomes with phospholipase C or S-adenosyl-L-methionine (SAM), or incubation in the presence of 2 mol NaCl/l did not increase the specific insulin binding. Therefore, the binding properties are similar to those observed in mammalian insulin-responsive tissues except for the absence of the effects of 2 mol NaCl/l, phospholipase C or SAM, which suggests the absence of masked insulin-binding sites.

J. Endocr. (1986) 108, 157–162