Inhibition of alkaline phosphatase activity by glucose

Non-enzymatic glycosylation (NEG) of alkaline phosphatase (AP) was studied after short- and long-term incubation with glucose and other carbohydrates. Glucose and amino sugars clearly inhibited the enzyme activity; this was in contrast to reducing and non-reducing disaccharides, which had an enhancing effect. After AP had been incubated with 18 nmol/l glucose for 180 minutes (short-term incubation), a subsequent extensive dialysis revealed full recovery of the enzymatic activity. This, plus the demonstration of a [3H]sodium borohydride-reducible glucose-protein adduct, indicated that initially a labile aldimine (Schiff base) had been formed. Binding experiments with [14C]glucose and failure of dialysis to achieve a recovery of enzymatic activity after long-term incubation suggested that subsequently a stable ketoamine product had been formed. This was further confirmed by the thiobarbituric acid test, which revealed 0.65 nmol 5-hydroxymethylfurfural/mg protein for glycosylated AP compared to 0.11 for the non-glycosylated control. Preliminary results further suggest that NEG of AP also occurs in vivo. Streptozotocin diabetic rats had significantly lower serum AP activities than did non-diabetic controls (mean +/- SD: 153.7 +/- 28.4 vs. 760.5 +/- 95.7 U/l; p less than 0.001). Blood glucose levels and serum AP activity, which had been determined simultaneously during an oral glucose tolerance test, showed without exception an inverse relationship in each of 32 healthy children studied. The biological significance of these findings remains to be established.