Insulin binding and internalization in hagfish red blood cells

Area of Science:

• Comparative endocrinology
• Cellular biology
• Biochemistry

Background:

• Insulin receptors are crucial for glucose homeostasis.
• Understanding insulin binding in primitive vertebrates like hagfish offers evolutionary insights.

Purpose of the Study:

• To characterize the binding kinetics and properties of porcine insulin in hagfish red blood cells.
• To investigate the specificity and affinity of insulin receptors in hagfish.

Main Methods:

• Radioligand binding assays using 125I-insulin.
• Competition studies with unlabeled insulin and other hormones.
• Scatchard analysis to determine binding site affinity.
• Internalization and degradation studies.

Main Results:

• Specific binding of 125I-insulin to hagfish red blood cells reached equilibrium within 1 hour at 10°C.
• Binding was reversible, with unlabeled insulin accelerating dissociation.
• Porcine insulin and desoctapeptide insulin competed for binding, but glucagon and somatostatin did not.
• Scatchard analysis indicated multiple affinity binding sites (high-affinity Ka = 0.2 x 10^9 M-1, low-affinity Ka = 0.27 x 10^7 M-1).
• Approximately 2090 binding sites per cell were calculated.
• 62% of bound 125I-insulin was internalized.
• Less 125I-insulin degradation was observed compared to human red blood cells.

Conclusions:

• Hagfish red blood cells possess specific, high- and low-affinity insulin binding sites.
• Insulin internalization occurs in hagfish red blood cells.
• Insulin binding characteristics in hagfish differ from those in human red blood cells, suggesting distinct evolutionary adaptations.