Method of determining glucose-6-phosphate and composition therefor

In a method for quantitatively determining a physiologically active substance capable of producing glucose-6-phosphate in a sample, comprising the steps of:

• (I) bringing said sample into contact with a composition of (i) one or more enzymes and reagents capable of quantitatively producing glucose-6-phosphate from said physiologically active substance, (ii) glucose-6-phosphate dehydrogenase and (iii) NAD or NADP, to thereby effect in the presence of said physiologically active substance, an ordered sequence of reactions wherein
• (a) glucose-6-phosphate is quantitatively produced from said physiologically active substance;
• and,
• (b) glucose-6-phosphate and NAD or NADP are converted to 6-phosphogluconate and NADH or NADPH;
• and then,
• (II) quantitatively detecting NADH or NADPH produced in said step (I)(b), the improvement which comprises adding to said composition from 0.1 to 50 u/ml of 6-phosphogluconolactonase to accelerate the conversion of glucose-6-phosphate and NAD or NADP to 6-phosphogluconate and NADH or NADPH in the reaction (b) whereby the accuracy for determining said physiological active substance is optimized by the presence of 6-phosphogluconolactonase whereby a linear relationship of the absorbance of said NADH or NADPH to the concentration of said substance is maintained.