2-Hydroxy-3-butynoate is both a substrate and an irreversible inactivator of the flavoenzyme L-lactate oxidase. The partitioning between catalytic oxidation of 2-hydroxy-3-butynoate and inactivation of the enzyme is determined by the concentration of the second substrate, 0 2 . Rapid reaction studies show the formation of an intermediate which is common to both the oxidation and inactivation pathways. This intermediate appears to be a charge-transfer complex between enzyme-reduced flavin and 2-keto-3-butynoate. It is characterized by a long-wavelength absorbing band ( A, , , 600 nm) and lack of fluorescence, making it easily distinguished from The use of acetylenic substrate analogues as specific irreversible inactivators of target enzymes was initiated by Bloch and his colleagues (Bloch, 1969), using 3-decynoyl thiolesters to inactivate /3-hydroxydecanoyl thiolester dehydrase. Specificity devolved from the fact that the enzyme converts the acetylenic molecule into a conjugated allenic thiolester, the actual alkylating agent, a t its active site. Subsequently, acetylenic analogues have been used to inactivate thiolase (Holland et al., 1973), plasma amine oxidase (Hevey et al., 1973), ketosteroid isomerase (Batzold and Robinson, 1 9 7 9 , and several pyridoxal phosphate-dependent enzymes (Abeles and Walsh, 1973;Marcotte and Walsh, 1975). In each instance carbanionic intermediates are suspected during catalysis, and covalent modification of some amino acid residue on the protein (apoprotein) is indicated.In addition several flavoenzymes can be induced to undergo irreversible inactivation by acetylenic analogues. Pargyline (N-benzyl-N-methylpropynylamine), long known to inhibit the flavin-linked monoamine oxidase (Hellerman and Erwin, 1968), appears to modify the FAD cofactor (Chuang et al., 1974). With N-dimethylpropynylamine as substrate, a covalent adduct forms a t N S of the flavin coenzyme (Maycock et al., 1976). In 1972 we reported the preliminary observation that the Mycobacterium smegmatis L-lactate oxidase is irreversibly inactivated by 2-hydroxy-3-butynoate due to covalent