The production and characterization of covalent amyloglucosidase-antibody conjugates using anti-human serum albumin immunoglobulin G are described. The conjugation procedure is based on the periodate oxidation of carbohydrate moieties that are covalently linked to the enzyme, followed by Schiff's base formation with amino residues on IgG. An ultrasensitive enzyme cycling assay for glucose, the product of maltose cleavage by amyloglucosidase, was developed in order to increase the sensitivity of detecting the enzyme-antibody conjugate. The cycling assay, which allows the accurate measurement of glucose in the picomole range, involves an enzymatic conversion of glucose to glucose-6-phosphate and then isomerization to fructose-6-phosphate. A futile cycle between fructose-6-phosphate and fructose-1,6-diphosphate results in accumulation of adenosine diphosphate at a rate proportional to the original glucose concentration. The rate was monitored by a spectrophotometric system involving pyruvate kinase, phospho(enol)pyruvate, lactate dehydrogenase, and diphosphopyridine nucleotide. © 1981.
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