Characterization of the insulin-binding sites in turtle thyroid microsomes

Area of Science:

• Endocrinology
• Comparative Physiology
• Molecular Biology

Background:

• Insulin's role in metabolic regulation is well-established in mammals.
• Understanding insulin binding in non-mammalian species provides insights into evolutionary conservation of hormone action.

Purpose of the Study:

• To investigate the specific binding characteristics of insulin to turtle thyroid microsomes.
• To determine if turtle thyroid insulin binding sites share similarities with mammalian tissues.

Main Methods:

• Radio-labelled insulin binding assays performed on turtle thyroid microsomes.
• Experiments evaluated binding dependence on time, temperature, pH, and protein concentration.
• Competition assays with unlabelled insulin, pro-insulin, glucagon, and growth hormone were conducted.
• Scatchard plot analysis and dissociation kinetics were assessed.
• Effects of trypsin, phospholipase C, S-adenosyl-L-methionine (SAM), and NaCl on binding were examined.

Main Results:

• Specific insulin binding to turtle thyroid microsomes was demonstrated, dependent on time, temperature, pH (optimum 8.0), and protein concentration.
• Unlabelled insulin and pro-insulin competed for binding, correlating with biological activity; glucagon and growth hormone did not.
• Scatchard analysis revealed high- and low-affinity binding sites, suggesting negative co-operativity.
• Trypsin abolished binding, while phospholipase C, SAM, or high salt concentrations did not enhance binding, unlike in some mammalian tissues.

Conclusions:

• Turtle thyroid microsomes contain specific, functional insulin binding sites.
• The binding characteristics are largely comparable to mammalian insulin-responsive tissues, with notable differences in the absence of masked binding sites sensitive to phospholipase C, SAM, or salt.