New methods for in vivo studies of hepatic metabolism

In vivo studies of liver metabolism have been limited for a long time to measurements, by the balance technique or the isotope dilution method, of the amounts of substrates taken up or produced by liver. New methods have been developed that now permit us to obtain important information on intrahepatic metabolic pathways. Nuclear magnetic resonance permits noninvasive studies of liver glycogen synthesis and breakdown. Chemical biopsy of glucuronic acid by acetaminophen also permits the study of glycogen synthesis whereas chemical biopsies of liver glutamine by phenylacetate and of cytosolic acetyl-CoA by sulfamethoxazole give important information concerning, respectively, Krebs cycle activity and gluconeogenesis and on lipogenesis and cholesterol synthesis. Mass isotopomer distribution analysis of molecules synthesized during the infusion of a deuterium or C-13-labeled precursor permits the estimation of in vivo gluconeogenesis as well as cholesterol synthesis and lipogenesis. Finally, these metabolic pathways can be studied through the incorporation of deuterium from deuterated water in glucose, fatty acids and cholesterol. All these noninvasive techniques will allow investigations to be undertaken in humans, addressing the nutritional and hormonal regulation of liver metabolism in normal subjects and in pathological situations.