| Abstract: | The kinetic properties of the rat intestinal microsomal 1-naphthol:UDPglucuronosyltransferase (EC 2.4.1.17) were investigated in fully activated microsomes prepared from isolated mucosal cells. The enzyme appeared to follow an ordered sequential bireactant mechanism in which 1-naphthol and UDP-glucuronic acid (UDPGlcUA) are the first and second binding substrates and UDP and 1-naphthol glucuronide the first and second products, respectively. Bisubstrate kinetic analysis yielded the following kinetic constants: Vmax = 102 +/- 6 nmol/min per mg microsomal protein, Km (UDPGlcUA) = 1.26 +/- 0.10 mM, Km (1-naphthol) = 96 +/- 10 microM and Ki (1-naphthol) = 25 +/- 7 microM. The rapid equilibrium random or ordered bireactant mechanisms, as well as the iso-Theorell-Chance mechanism, could be excluded by endproduct inhibition studies with UDP.UDP-N-acetylglucosamine (UDPGlcNAc), usually found to be an activator of UDP glucuronosyltransferase in liver microsomes, acted as a full competitive inhibitor towards UDPGlcUA in rat intestinal microsomes. With regard to 1-naphthol UDPGlcNAc exhibited a dual effect: both inhibition and activation was observed. The effect of activation by MgCl2 and Triton X-100 on the kinetic constants and the inhibition patterns of UDP and UDPGlcNAc were investigated. The results obtained suggest that latency in rat intestinal microsomes may be due to endproduct inhibition by UDP. This endproduct inhibition could be abolished by in vitro treatment with MgCl2 and Triton X-100. |
