Patent ID: 6902719

Claim:
A method of assaying enzyme activity in a subject, said method comprising: a) administering to a subject an effective amount of a reverse tracer wherein said reverse tracer is a carbon labeled molecule that is constitutively metabolized by the subject to produce a labeled metabolite wherein said carbon labeled molecule is selected from the group consisting of acetate, glucose, bicarbonate, glycine, octanoate, palmitate, formate, propionate, and urea; b) administering to said subject an effective amount of an unlabeled substrate, wherein said substrate is specifically metabolized by an enzyme to be assayed and wherein said substrate is metabolized by said enzyme to produce and unlabeled metabolite that is the same as the metabolite from step a); c) collecting a specimen from said subject; and d) measuring the amount of labeled metabolite in said specimen to determine the activity of said enzyme in said subject wherein the dilution of labeled metabolite indicates enzyme activity.