IMPAIRMENT OF UDP-GLUCOSE DEHYDROGENASE AND GLUCURONIDATION ACTIVITIES IN LIVER AND SMALL-INTESTINE OF RAT AND GUINEA-PIG IN-VITRO BY PIPERINE

The effects of piperine, a major ingredient of black pepper, on UDP-glucose dehydrogenase (UDP-GDH) and glucuronidation potentials of rat and guinea pig liver and intestine were studied. Piperine caused a concentration-related strong inhibition of UDP-GDH (50% at 10 muM) reversibly and equipotently, in both tissues. Partially purified rat liver UDP-GDH was used to obtain the kinetic values at pH optima of 9.4 and 8.6. At pH 9.4: K(m)UDP-glucose = 15 muM, V(max) = 5.2 nmol NADH/min/mg protein, K(i) = 6 muM. With NAD, a K(i) of 16 muM was obtained. At pH 8,6: K(m) = 35 muM, V(max) = 7.5 nmol, K(i) = 15 muM. In all of these cases, piperine caused non-competitive inhibition. Data from structure-activity comparisons of piperine analogs indicated that the presence of conjugated double bonds in the side chain of the molecule is a factor in piperine inhibition. However, the UDP-glucuronic acid (UDPGA) contents were decreased less effectively by piperine in isolated rat hepatocytes compared with enterocytes of guinea pig small intestine. Piperine at 50 muM caused a marginal decrease of UDPGA in hepatocytes when the rate of glucuronidation of 3-hydroxybenzo[a]pyrene (3-OH-BP) decreased by about 40%. The decrease obtained at 10 muM piperine in intestinal cells was comparable to that obtained at 50-100 muM in hepatocytes. UDP-glucuronosyltransferase (UGT) activities towards 3-OH-BP (UGT1A1) and 4-OH-biphenyl (UGT2B1) were also determined. Piperine did not affect the rate of glucuronidation of 4-OH-biphenyl in rat liver, whereas that of 3-OH-BP was impaired significantly. In guinea pig small intestine, both these activities were inhibited significantly requiring less than 25 muM piperine to produce a more than 50% inhibition of UGT(s). The results suggested that (i) piperine is a potent inhibitor of UDP-GDH, (ii) inhibition is offered exclusively by the conjugated double bonds of the molecule, and (iii) piperine exerts stronger effects on intestinal glucuronidation than in rat liver.