Molecular and CellularBiochemistry 101: 157-166, 1991. © 1991 Kluwer Academic Publishers. Printed in the Netherlands. Study of properties of cholinephosphotransferase from fetal guinea pig lung mitochondria and microsomes* Sukla Ghosh, Patricia W. Oten, Shymali Mukherjee and Salil K. Das Department of Biochemistry, Meharry Medical College, Nashville, TN 37208, USA Received27 March 1990;accepted20 July 1990 Key words: cholinephosphotransferase, mitochondria, microsomes, fetal lung, guinea pig Summary We have reported earlier that cholinephosphotransferase (EC 2.7.8.2) is present in both mitochondria and microsomes of fetal guinea pig lung. This study was designed to compare the properties of mitochondrial and microsomal cholinephosphotransferase in fetal guinea pig lung. Various parameters, such as substrate specificity, K m values, sensitivity to N-ethylmaleimide, dithiothreitol and trypsin were measured. Both showed significant preference for unsaturated diacylglycerols over saturated diacylglycerols. Data on K m and Vmaxindicate that the affinity of this enzyme for different diacylglycerols varies between the two forms. The IDs0 values for N-ethylmaleimide were 20 mM and 12.5 mM for the mitochondrial and microsomal form of the enzyme, respectively. Dithiothreitol showed an inhibitory effect on both; however, the mitochondrial form was inhibited less than the microsomal form. The effects of N-ethylmaleimide and dithiothreitol on both forms of enzyme indicated that the microsomal cholinephosphotransferase requires a higher concentra- tion of -SH for its activity than the mitochondrial enzyme does. The enzyme was inhibited by trypsin in both mitochondria and microsome under isotonic condition suggesting that this enzyme is on the outside of the membrane in both endoplasmic reticulum and mitochondria. Introduction The tung becomes functional at birth, when it un- dergoes a transition from a liquid to gas filled envi- ronment. This transition can not occur successfully in the absence of a surfactant system in alveolar type II cells. Pulmonary surfactant is a lipoprotein in which the lipid fraction consists predominantly of phosphatidylcholine, over 60% of which is sat- urated [1]. This saturated phosphatidylcholine is largely dipalmitoylphosphatidylcholine [2, 3]. Many investigators have studied the pathways in- volved in the formation of pulmonary lipids, with particular emphasis on the biosynthesis of dipalmi- toylphosphatidylcholine [4]. It is now generally be- lieved that surfactant dipalmitoylphosphatidylcho- line can be directly synthesized de novo via CDP- choline pathway [5-9] or by a concerted action between synthesis de novo and remodelling of un- saturated phosphatidyl choline [10]. Even though cholinephosphocytidyltransferase is known to catalyse the rate limiting step in the CDP-choline pathway in type II cells [11] and other cells [12] in providing CDP-choline, cholinephos- photransferase plays also a regulatory role since it *Presented in part at the Seventy-FourthAnnual Meeting of the Federation of American Societies for Experimental Biology, Washington D.C., April 1990.