Identification and characterization of dehydrocholic acid reductase system in the cytosol of human red blood cells.
We conducted in vivo and in vitro studies of the reductive metabolism of the cholagogue, dehydrocholic acid (DHCA). Immediately after the intravenous administration of 1 g of DHCA in normal subjects (n = 6), the concentration of the reductive metabolite, 3 alpha-hydroxy-7,12-dioxo-cholanoic acid (unconjugated form), increased sharply in the systemic circulation, rising to 95.8 microM 10 min after administration. The results of in vitro experiments with DHCA and whole blood showed that 3 alpha-hydroxy-7,12-dioxocholanoic acid were produced from DHCA. In vitro experiments using DHCA and the red blood cell fraction, and DHCA and the red blood cell cytoplasmic fraction gave similar results to those described above with whole blood. However, a reductive metabolite was not formed by the incubation of DHCA and the red blood cell membrane fraction. These findings indicated that, contrary to the conventional theory that intravenously administered DHCA is subjected to reductive metabolism only in the liver, reduction also occurs in the systemic circulation, and the mechanism for this reductive metabolism is present in the cytoplasmic fraction of red blood cells. Further investigation to characterize this reductive metabolic system revealed an optimum temperature of 37 degrees C, an optimum pH of 7.4, a Km value of 2.0 x 10(-3) M, and inactivation by heat treatment (70 degrees C for 2 min).