ISOLATION OF AN ENZYME-BOUND INTERMEDIATE By J. BAR-TANA, G. ROSE and B. SHAPIRO

The overall reaction catalysed by purified rat liver palmitoyl-CoA synthetase (EC 6.2.1.3) apparently proceeds via an enzyme-bound intermediate that may be either enzyme-fatty acyl-AMP (Berg, 1956), palmitate-enzyme-AMP, palmitate-enzyme-PP, or enzyme-palmitate. This conclusion is consistent with the following observations (Bar-Tana et al., 1973): (a) the initial velocity pattern of the overall forward reaction conforms to either the Bi Uni Uni Bi Ping Pong (Cleland, 1963) or the Bi Bi Uni Uni Ping Pong mechanism, in which both ATP and palmitate have to combine with the enzyme before any product is released; (b) 180 is transferred from [carboxy-180]palmitate to AMP and palmitoyl-CoA and not to pyrophosphate; (c) 'pulse'-labelling of the enzyme with radioactive palmitate occurs only in the presence of ATP. The distinction between the alternatives presented seemed to be of interest, since no partial reactions using synthetic palmitoyl-AMP as substrate, to yield ATP or palmitoyl-CoA in the presence ofPPi or CoA respectively, were found to be catalysed by the purified enzyme (Bar-Tana et al., 1972). In the present paper the isolation of the enzyme-bound intermediate is described and its nature is discussed.