MECHANISM OF ACTION OF A WOUND-INDUCED OMEGA-HYDROXY-FATTY ACID - NADP OXIDOREDUCTASE ISOLATED FROM POTATO-TUBERS (SOLANUM-TUBEROSUM L)

ω-Hydroxyfatty acid:NADP oxidoreductase, an enzyme involved in suberin biosynthesis, is induced by wounding potato tubers. Initial velocity and product inhibition studies with the purified enzyme suggested an ordered sequential mechanism, where NADPH is added first, followed by 16-oxohexadecanoate, and NADP is released after 16-hydroxyhexadecanoate. Substrate inhibition by NADPH was observed at concentrations higher than 0.2 mm. The inhibitory NADPH molecule competes with 16-oxohexadecanoate, indicating that it forms a dead-end complex with the E-NADPH form of the enzyme. The kinetics for the NADPH inhibition suggested that n > 1 in the rate equation v = V[NADPH] (Km + [NADPH]+ [NADPH]n+1 Ki); i.e., more than two NADPH molecules bind to enzyme. The Km for 16-oxohexadecanoate did not change from pH 7.5 to 9.0 but increased about 10-fold from pH 9.0 to 10.0, whereas the Km for NADPH and hexadecanal did not vary significantly in this pH range. Phenylglyoxal inactivated the enzyme; NADPH and AMP (which competes with NADPH; Ki = 1.1 mM) provided protection against such inactivation. Diethylpyrocarbonate also caused inactivation which was reversed by hydroxylamine; NADPH but not AMP protected the enzyme from this inhibition. Pyridoxal-5′-phosphate reversibly inactivated the enzyme and NaBH4 reduction of the pyridoxal phosphate-treated enzyme resulted in irreversible inhibition; a combination of NADPH and ω-oxo C16 acid provided protection against such inactivation. As the chain length of alkanals increased from C3 to C8, the Km for the substrate decreased drastically from 7000 to 90μm and a further increase in chain length from C8 to C20 resulted in only a small decrease in Km. The Km and V for 8-oxooctanoate and 10-oxodecanoate are compared with the values obtained for 16-oxohexadecanoate. Based on these results, it is proposed that arginine acts as the binding site for NADPH, a hydrophobic crevice with lysine at the bottom forms the binding site for 16-oxohexadecanoate and histidine participates in the reaction as the proton donor. © 1978.