Glycosylation of plasma proteins was studied in diabetic and normal subjects by an adaptation of a thiobarbituric acid method previously used for glycosylated hemoglobin. There was a highly significant correlation between the degree of glycosylation in vivo of plasma protein and hemoglobin. The process of glycosylation depended on time and temperature and was not mediated enzymatically. Plasma protein of all molecular sizes could be glycosylated. At 37°C, 4.6% of 14C-glucose became attached to plasma protein after 24 h of incubation in vitro; no difference between normal and diabetic plasma protein could be demonstrated. After glycosylation, glucose dissociated from protein slowly; 72% remained attached after dialysis for 24 h. As an estimate of diabetic control, measurement of glycosylation of plasma protein is a suitable alternative to determination of glycosylated hemoglobin. Results of each method correlated equally well with the degree of diabetic control when this was assessed by calculation of the M-factor, mean plasma glucose concentrations, or variance of the plasma glucose determinations. In hemolytic anemia, hemoglobinopathy, and recent transfusion, measurement of glycosylation of plasma protein may be more accurate than that of glycosylated hemoglobin. It may also provide an estimate of diabetic control between that provided by short-term blood glucose determination and long-term glycosylated hemoglobin.

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