Inhibitory effects of adenine nucleotides and related substances on UDP-glucuronosyltransferase: structure-effect relationships and evidence for an allosteric mechanism.
The inhibitory effects of nucleotides and related substances on rat hepatic UDP-glucuronosyltransferase (UGT) were studied. ATP and NADP+ markedly reduced 4-methylumbelliferone (4-MU) UGT activity only when detergent-treated rat liver microsomes were used as the enzyme source. The IC50 values of adenine, ATP, NAD+ and NADP+ were estimated to be below 20 microM, whereas AMP had no inhibitory effect. From the kinetic behavior observed, these adenine-related compounds were assumed to inhibit UGT activity non-competitively without competing with either 4-MU or UDP-glucuronic acid. Among guanine, cytosine and their related nucleotides, only triphosphate nucleotides (CTP and GTP) exhibited potent UGT inhibition, although the effect of GTP was weak. Estradiol 3- and 17-glucuronidation were also inhibited by the inhibitors of 4-MU UGT. The only exception was that estradiol 17-glucuronidation activity was inhibited by AMP (IC50=31 microM). In addition, AMP antagonized the inhibitory effects of adenine, ATP, and NADP+ on 4-MU and estradiol 3- glucuronidation activities. These results suggest that (1) a number of cellular nucleotides present within the endoplasmic reticulum regulate UGT function; and (2) these substances bind to a common allosteric site on UGT to reduce catalytic function.
Nishimura
Yoshio
Y
Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.
Maeda
Shingo
S
Ikushiro
Shin-ichi
S
Mackenzie
Peter I
PI
Ishii
Yuji
Y
Yamada
Hideyuki
H
D000227:Adenine Nucleotides / Q000737:chemistry / Q000502:physiology*; D000494:Allosteric Regulation / Q000502:physiology; D000818:Animals; D002460:Cell Line; D014453:Glucuronosyltransferase / Q000037:antagonists & inhibitors* / Q000096:biosynthesis / Q000235:genetics; D006801:Humans; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology; D051381:Rats; D017208:Rats, Wistar; D013329:Structure-Activity Relationship; D014530:Uridine Diphosphate / Q000737:chemistry / Q000502:physiology
D000227:Adenine Nucleotides; D014530:Uridine Diphosphate; D014453:Glucuronosyltransferase