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Plant Physiology 77:12-15 (1985)
© 1985 American Society of Plant Biologists

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Biosynthesis of Cytidine 5'-Diphosphate-diacylglycerol in Endoplasmic Reticulum and Mitochondria of Castor Bean Endosperm 1

Kathryn F. Kleppinger-Sparace2 and Thomas S. Moore, Jr.

Arco Plant Cell Research Institute, 6560 Trinity Court, Dublin, California 94568, Department of Botany, Louisiana State University, Baton Rouge, Louisiana 70803

Cytidine 5'-triphosphate (CTP):phosphatidate cytidyltransferase from the endoplasmic reticulum and mitochondria of Ricinus communis L. var Hale was characterized. The endoplasmic reticulum enzyme has a pH optimum of 6.5 and a divalent cation is required, Mn2+ being preferred and giving maximum activity at 2.5 millimolar. The estimated Km for CTP is 16.7 micromolar, but that for phosphatidate could not be determined accurately. The activity was inhibited by both deoxycholate and Triton X-100 at concentrations as low as 0.01% (w/w).

The mitochondrial enzyme has a pH optimum of 6.0 and a divalent cation requirement similar to that of the endoplasmic reticulum. Maximum stimulation of the reaction by substrates occurred with 1.5 millimolar phosphatidate (from egg phosphatidylcholine) and about 400 micromolar CTP. The apparent Km for phosphatidate could not be estimated accurately since activity was obtained in the absence of added lipid, apparently utilizing endogenous substrate. The Km estimated for CTP was altered by the presence of the detergent Triton X-100; in its absence the value was 33.3 micromolar, but in its presence the value was 66.7 micromolar. Inclusion of 0.6% (w/w) Triton X-100 in the assay mixture stimulated the activity about 2.5-fold.

2 A portion of this research was submitted by K.F.K.-S. to the University of Wyoming to fulfill part of the requirement for the M.S. degree.

1 Supported by grants PCM-8025006 and PCM78-06817 from the National Science Foundation.






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Copyright © 1985 by the American Society of Plant Biologists