Mechanism of the reaction catalyzed by acyl-CoA: lysolecithin acyltransferase from rabbit lung. pH studies and chemical modification.

This paper deals with the first attempt to elucidate the chemical mechanism of acyl-CoA: lysolecithin acyltransferase from rabbit lung, a key enzyme in the metabolism of lung surfactant. For this purpose, the pH dependence of kinetic constants as well as the chemical modification of the protein have been studied on a partially-purified preparation. From these experiments, the pKs on which the activity of the enzyme relies have been calculated, giving values of pK1 congruent to 5.5 and pK2 congruent to 10. Analysis of the effect of organic solvents on these pKs and the calculation of the enthalpies of ionization, together with the chemical modification experiments, lead to the conclusion that pK1 is due to an histidine residue, whereas pK2 arises from the amino group of the adenine ring of palmitoyl-CoA. Moreover, chemical modification demonstrated an essential cysteine. A tentative chemical mechanism, in accordance with these results, is proposed and it is hypothesized, in view of other results obtained in our laboratory and from the literature, that the chemical mechanism of acyl transfer to sn-2 position may be common to other enzymes of glycerolipid metabolism.