Kinetics of proinsulin conversion in human islets.
Islets isolated from human cadaver pancreas were pulse-labeled (10 min with [3H]leucine) and then incubated for a 180-min chase. Islets and chase medium were collected every 15 min and analyzed by reversed-phase HPLC to quantify the percentage of radioactively labeled proinsulin, conversion intermediates, and fully processed insulin. Release of proinsulin-related labeled products into the chase medium was < 10% of total. Whereas 50% of labeled proinsulin had been lost by conversion within 45 min, fully processed insulin only appeared with a half-time of 100 min. This discrepancy is attributable to accumulation of radioactive conversion intermediates. Des 64.65 split proinsulin was a minor component, reaching a maximum of 5.2 +/- 1.7% (n = 4) at 60 min of chase. By contrast, des 31.32 split proinsulin--and a truncated form lacking the first three residues of C-peptide--rose progressively to 29.3 +/- 1.4% by 75 min, and declined thereafter. The accumulation of des 31.32 split proinsulin rather than the des 64.65 split form during the conversion of human proinsulin reflects slower conversion at the C-peptide/A-chain than at the B-chain/C-peptide junction, and is consistent with the appearance of this particular conversion intermediate in the circulation.
Laboratoires de Recherche Louis Jeantet, Centre Médical Universitaire, Geneva, Switzerland.
S
S
Sizonenko
J C
JC
Irminger
L
L
Buhler
S
S
Deng
P
P
Morel
P A
PA
Halban
D000328:Adult; D000595:Amino Acid Sequence; D002096:C-Peptide / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism; D007515:Islets of Langerhans / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008875:Middle Aged; D008969:Molecular Sequence Data; D011384:Proinsulin / Q000378:metabolism*; D013997:Time Factors
D002096:C-Peptide; D007328:Insulin; D011384:Proinsulin