Insulin biosynthesis in the bullhead, Ictalurus nebulosus, and the effect of temperature
Insulin biosynthesis in the brown bullhead, Ictalurus nebulosus (Le Sueur), was studied by measuring the incorporation in vitro of [3H]leucine into proteins of the principal islet. The tissue was incubated for 6–15h in Krebs–Ringer bicarbonate buffer with [3H]leucine, supplemented with amino acids and glucose. Proteins, precipitated with trichloroacetic acid and extracted with acid ethanol, were separated by gel-filtration on Biogel P-30 in 3m-acetic acid. Three major components were found after incubation of the islets at 22°C. On the basis of the results of sulphitolysis, biological activity and the demonstrated precursor–product relationship, components I and II were identified as proinsulin and insulin respectively. The third component was not identified. At 12°C, [3H]leucine was incorporated only into proinsulin. No radioactivity was found in insulin or the unidentified component III at 12°C as was found after incubation at 22°C. When the temperature was lowered from 22° to 12°C after 3h of a 15h incubation, decreased conversion of proinsulin into insulin resulted at the lower temperature compared with the control tissue maintained at 22°C. When the temperature was raised from 12° to 22°C at 3h of a 15h incubation, conversion of proinsulin into insulin occurred. No conversion occurred in the control tissue with the temperature maintained at 12°C. No qualitative difference in the incorporation of [3H]leucine into proinsulin and its conversion into insulin at 12° and 22°C could be demonstrated between islet tissue from fish acclimated to less than 12°C or to 22°C. The results suggest that the enzyme(s) responsible for converting proinsulin into insulin in the bullhead may be temperature sensitive with low activity at 12°C.