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Incubation of rat serum with D-glucose in vitro resulted in nonenzymatic glucosylation of serum proteins. Analysis of freshly isolated rat albumin by ion exchange chromatography indicated that the glucosylated albumin accounts for 6.7.+-. 0.9% of total albumin in normal rat serum. Glucosylation of rat albumin in vitro was 1st order with respect to glucose and albumin concentrations and occurs primarily (> 90%) at intrachain lysine residues. Kinetic analysis and inhibition of glucosylation by aspirin suggest that 1 reactive lysine residue is the primary site of glucosylation. Less than 5% of the radioactivity from glucosyl-albumin was released as glucose or mannose by hydrolysis conditions normally used for the analysis of neutral sugars in glycoproteins. Studies in vivo demonstrated that the half-life of albumin in normal rats was unaffected by the addition of 1 mol of glucose/mol of albumin. In addition, glucosylation was a stable modification since 125i-albumin isolated up to 3 days after injection of glucosylated 125i-albumin was recovered only in the glucosylated fraction. In contrast, following injection of unglucosylated 125i-albumin there was a gradual shift of 125i radioactivity to the glucosylated albumin fraction, as would be predicted for nonenzymatic glucosylation occurring in the circulation. Finally, levels of glucosylated albumin isolated from diabetic rats (alloxan induced) were significantly (4-fold) elevated 4 days after withdrawal from insulin therapy. The rat should be a suitable animal model for in vivo studies on nonenzymatic glucosylation of albumin and other serum proteins in normal and diabetic metabolic states.

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