The intracellular handling of insulin-related peptides in isolated pancreatic islets. Evidence for differential rates of degradation of insulin and C-peptide.
Islets of Langerhans isolated from adult rats were maintained in tissue culture for 3 days in the continued presence of [3H]leucine. Labelled proinsulin, C-peptide and insulin were measured by quantitative h.p.l.c., a method which also allowed for resolution of C-peptide I and II, and of insulin I and II (the products of the two rat insulin genes). The results showed that: (1) at early times, proinsulin was the major radiolabelled product; with progressive time in culture, intra-islet levels of [3H]proinsulin decreased, despite continuous labelling with [3H]leucine, indicating that the combined rates of proinsulin conversion into insulin and of proinsulin release, exceeded the rate of synthesis; (2) insulin I levels were always greater than those of insulin II, both in the islets and for products released to the medium; (3) the molar ratio of [3H]insulin I and II to their respective 3H-labelled C-peptides increased with time for products retained within islets, reaching a value close to 3:1 by 3 days; by contrast, for products released to the medium during the culture period, the ratio was always close to unity; (4) when islets were incubated with [3H]leucine for 2 days, and then left for a further 1 day without label (chase period), the intra-islet [3H]insulin/[3H]C-peptide ratios rose to values as high as 9:1. Again, for material released to the medium, the values were close to 1:1; (5) it is concluded that C-peptide is degraded more rapidly than insulin within islet cells, thereby accounting for the elevated insulin/C-peptide ratios. The difference between the ratios observed in the islets and those for material released to the medium is taken to indicate that degradation occurs in a discrete cellular compartment and not in the secretory granule itself.
Rhodes
C J
CJ
E.P. Joslin Research Laboratory, Joslin Diabetes Center, Brigham and Women's Hospital, Boston, MA 02215.
Halban
P A
PA
D000818:Animals; D002096:C-Peptide / Q000378:metabolism*; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D007328:Insulin / Q000378:metabolism*; D007424:Intracellular Fluid / Q000378:metabolism; D007515:Islets of Langerhans / Q000378:metabolism*; D007930:Leucine / Q000378:metabolism; D008297:Male; D011384:Proinsulin / Q000378:metabolism; D011863:Radioimmunoassay; D051381:Rats; D011919:Rats, Inbred Strains
D002096:C-Peptide; D007328:Insulin; D011384:Proinsulin; D007930:Leucine