Determination of metabolites by observing the change in reaction rate during the early course of a first order reaction, as applied to fructose-1 phosphate.

F-I-P’ is an intermediary metabolite, which is accumulated during the metabolism of fructose in the liver (4,7). Enzymatic determination of F-l-P in tissue extracts has previously been made with either rat liver aldolase (EC 4.1.2.7) (2) or rabbit muscle F-6-PK (EC 4.1.2.13) (5). Determination with aldolase is more rapid than with F-6-PK, but this enzyme is not commercially available. Determination with F-6-PK takes advantage of the fact that this enzyme can phosphorylate F-l-P and fructose as well as F-6-P, though with activities of only 1 and < 0.15%, respectively, relative to that with F-6-P. F-6-PK has a very high Michaelis constant for F-l-P (K,,,.,) with the result that the time for the reaction to go to completion is long. With the method of Michal and Gruber (5) for the determination of F-I-P in liver extracts, assays can take 2-3 hr to perform. In this latter