A COMPARTMENTAL MODEL OF 5-METHYLHISTIDINE METABOLISM IN HUMANS

Urinary 3-methylhistidine (3MH) excretion has been proposed as a nonin vasive in vivo marker of muscle protein breakdown, but such analysis r equires quantitative collection of urine and yields few details about the metabolism of 3MH. In this study, we propose that data from a sing le bolus dose of tracer and serial blood samples over 72 h can be desc ribed by a kinetic model that defines 3MH metabolism in humans. Plasma concentration of the tracer was described by a linear time-invariant three-compartment model. The model defines masses and fluxes of 3MH wi thin the subjects and, in particular, the intracellular de novo produc tion of 3MH. The de novo production of 3MH as calculated by the model was not different from that calculated via the traditional collection of urinary 3MH (3.09 vs. 2.57 mu mol . kg(-1). day(-1), respectively; P > 0.30). These data indicate that 3MH production can be measured by a compartmental model that can be used to measure muscle proteolysis w ithout quantitative urine collections.