The contribution of newly synthesized cholesterol to bile salt synthesis in rats quantified by mass isotopomer distribution analysis

A new stable isotope procedure has been developed and validated in rats, ap plying [1-C-13]acetate infusion to quantify the production of bile salts fr om de novo synthesized cholesterol making use of the mass isotopomer distri bution analysis (MIDA) principle. Ions (m/z) 458-461, 370-373 and 285-288 w ere monitored by GC/MS (EI-mode) for the methyl trimethylsilylether derivat ives of cholate, chenodeoxycholate and beta-muricholate, respectively. Rats with intact exteriorized enterohepatic circulation and rats with chronic b ile diversion were infused with [1-C-13]acetate for up to 14 h. After 10 h of infusion the enterohepatic circulation of the intact group was interrupt ed to deplete the existing bile salt pool (acute bile diversion). The fract ions of biliary cholesterol and individual bile salts derived from newly sy nthesized cholesterol were determined by MIDA at t = 14 h. In rats with acu te bile diversion, these fractions were 20, 25, 27 and 23% for biliary chol esterol, cholate, chenodeoxycholate and beta-muricholate, respectively. Aft er bile diversion for 8 days to induce hepatic cholesterol and bile salt sy nthesis, these fractions increased significantly to 32, 47, 41 and 47%, res pectively. Calculated enrichments of the acetyl-CoA precursor pools were si milar for all bile salts and biliary cholesterol within the two rat groups. However, chronic enterohepatic interruption decreased the acetyl-CoA pool size almost two-fold. We conclude that MIDA is a validated new stable isoto pe technique for studying the synthetic pathway from acetyl-CoA to bile sal ts. This technique provides an important new tool for studying bile salt me tabolism in humans using stable isotopes. (C) 2000 Elsevier Science B.V. Al l rights reserved.