Evidence that unsaturated fatty acids are potent inhibitors of renal UDP-glucuronosyltransferases (UGT): kinetic studies using human kidney cortical microsomes and recombinant UGT1A9 and UGT2B7

Renal ischaemia is associated with accumulation of fatty acids (FA) and mobilisation of arachidonic acid (AA). Given the capacity of UDP-glucuronosyltransferase (UGT) isoforms to metabolise both drugs and FA, we hypothesised that FA would inhibit renal drug glucuronidation. The effect of FA (C2:0-C20:5) on 4-methylumbelliferone (4-MU) glucuronidation was investigated using human kidney cortical microsomes (HKCM) and recombinant UGT1A9 and UGT2B7 as the enzyme sources. 4-MU glucuronidation exhibited Michaelis-Menten kinetics with HKCM (apparent K(m) (K(m)(app)) 20.3 microM), weak substrate inhibition with UGT1A9 (K(m)(app) 10.2 microM, K(si) 289.6 microM), and sigmoid kinetics with UGT2B7 (S(50)(app)440.6 microM) Similarly, biphasic UDP-glucuronic acid (UDPGA) kinetics were observed with HKCM (S(50) 354.3 microM) and UGT1A9 (S(50) 88.2 microM). In contrast, the Michaelis-Menten kinetics for UDPGA observed with UGT2B7 (K(m)(app) 493.2 microM) suggested that kinetic interactions with UGTs were specific to the xenobiotic substrate and the co-substrate (UDPGA). FA (C16:1-C20:5) significantly inhibited (25-93%) HKCM, UGT1A9 or UGT2B7 catalysed 4-MU glucuronidation. Although linoleic acid (LA) and AA were both competitive inhibitors of 4-MU glucuronidation by HKCM (K(i)(app) 6.34 and 0.15 microM, respectively), only LA was a competitive inhibitor of UGT1A9 (K(i)(app) 4.06 microM). In contrast, inhibition of UGT1A9 by AA exhibited atypical kinetics. These data indicate that LA and AA are potent inhibitors of 4-MU glucuronidation catalysed by human kidney UGTs and recombinant UGT1A9 and UGT2B7. It is conceivable therefore that during periods of renal ischaemia FA may impair renal drug glucuronidation thus compromising the protective capacity of the kidney against drug-induced nephrotoxicity.