Diabetic BB/Wor rat haptoglobin exhibits a probable structural abnormality in Asn-linked oligosaccharides.
In this study we demonstrate that haptoglobin, a serum glycoprotein secreted by the liver, has altered structure in the BB/Wor diabetic rat. SDS-PAGE of haptoglobin (a tetramer composed of two glycosylated beta-chains each containing two sites for Asn-linked oligosaccharides connected by disulfide bonds with two nonglycosylated alpha-chains) clearly shows that the beta-chain of haptoglobin from diabetic rats is smaller than normal, with a molecular mass of 39 instead of 40 kDa. Both acute and chronic diabetic rats exhibit the defect. Defective haptoglobin appears in the serum within 4 days of onset of the disease, but insulin therapy prevents the defect. Removal of Asn-linked oligosaccharides with peptide: N-glycosidase F from Flavobacterium meningosepticum abolished the size difference between the beta-chains from normal and diabetic haptoglobin, with the molecular mass in both cases shifting to 30 kDa. Haptoglobin from both normal and diabetic rats was resistant to digestion by endoglycosidase H from Streptomyces griseus, which cleaves high mannose-type chains. Removal of sialic acid with neuraminidase treatment resulted in a reduction in the molecular mass in both cases, but without eliminating the size difference between the two. These results demonstrate that haptoglobin from diabetic BB/Wor rats contains a structural abnormality which correlates with onset of the disease. The defect is most likely due to an alteration in Asn-linked oligosaccharides, probably involving a change in the neutral sugars of complex-type oligosaccharide chains. This finding represents the first example of an altered Asn-linked oligosaccharides in diabetes.
Chapman
A E
AE
Department of Pediatrics, New England Medical Center, Boston, MA.
Copeland
P
P
Davidson
S
S
Calhoun
J C
JC
D000818:Animals; D001216:Asparagine; D001797:Blood Protein Electrophoresis; D003922:Diabetes Mellitus, Type 1 / Q000097:blood*; D004591:Electrophoresis, Polyacrylamide Gel; D006242:Haptoglobins / Q000032:analysis / Q000737:chemistry* / Q000378:metabolism; D009439:Neuraminidase / Q000494:pharmacology; D051381:Rats; D011913:Rats, Inbred BB
D006242:Haptoglobins; D001216:Asparagine; D009439:Neuraminidase