EFFECTS OF 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID AND OXYGEN CONCENTRATIONS ON IN-VIVO AND IN-VITRO ACTIVITY OF ACC OXIDASE OF SUNFLOWER HYPOCOTYL SEGMENTS

In vivo ethylene production by hypocotyl segments of sunflower seedlin gs and in vitro activity of 1-aminocyclopropane-1-carboxylic acid oxid ase (formerly ethylene-forming enzyme) extacted from the same tissues increase with increasing concentrations of 1-aminocyclopropane-1-carbo xylic acid (ACC) and oxygen. ACC oxidase activity follows Michaelis-Me nten kinetics. The apparent Km values of the enzyme towards ACC, estim ated in vivo and in vitro, are respectively 219 mu M and 20.6 mu M. Bo th Km values towards O-2 are similar, ca 10.6-11.4%. A decrease in con centration in one of the substrates (ACC or O-2) results in an increas e in in vivo apparent Km of ACC oxidase for the other substrate. On th e contrary, Km values of the enzyme towards ACC or O-2 estimated in vi tro are not dependent upon the concentration of the other substrate (A CC or O-2).