Plant 5-Methylthioribose Kinase

Abstract

Activity of 5-methylthioribose kinase, the enzyme which catalyzes the ATP-dependent formation of 1-phospho-5-methylthioribose, was revealed in the extracts from various higher plant species [Glycine max cv. Warszawska, Cucumis sativus cv. Eva, Helianthus annuus cv. Bronowski prazkowany, Zea mays cv. Pulawska zolta and Hordeum vulgare cv. Polon]. Almost 2000-fold-purified enzyme was obtained from yellow lupine (Lupinus luteus L. cv. Topaz) seed extract. The MW of the native enzyme is 70,000 as judged by gel filtration. The lupine 5-methylthioribose kinase exhibits a strict requirement for divalent metal ions. Among the ions tested, only Mg2+ and Mn2+ acted as cofactors. The curve of kinase initial velocity vs. pH reaches plateau at pH 10-10.5. The Km values calculated for 5-methylthioribose and ATP are 4.3 and 8.3 .mu.M, respectively. Among nucleoside triphosphates tested as potential phosphate donors, only dATP could substitute in the reaction for ATP. 5-Isobutylthioribose, an analog of 5-methylthioribose, was the .gamma.-ATP-phosphate acceptor. The compound competitively inhibits synthesis of 1-phospho-5-methylthioribose (Ki = 1.4 .mu.M). Lupine 5-methylthioribose kinase is completely and irreversibly inhibited by the antisulfhydryl reagent, p-hydroxymercuribenzoate. As in bacteria (Ferro et. al. 1978), the enzyme may be involved in a new, alternative pathway of methionine synthesis in plant tissues.