Warm growth temperatures decrease soybean cholinephosphotransferase activity.
The activity of cytidine 5'-diphosphate (CDP) choline: 1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) in developing soybean (Glycine max L. var Williams 82) seeds was 3 to 5 times higher in cotyledons grown at 20 degrees C than in those grown at 35 degrees C. Some characteristics of the enzyme from cotyledons cultured at 20 and 35 degrees C were compared. In preparations from both growth temperatures, the enzyme showed a pH optimum of 7, K(m) of 7.0 micromolar for CDP-choline, and an optimum assay temperature of 45 degrees C. Both enzyme preparations were stimulated by increasing concentrations of Mg(2+) or Mn(2+), up to 10 millimolar and 50 micromolar, respectively, though Mn(2+) produced lower activities than Mg(2+). Enzymes from both 20 and 35 degrees C show the same specificity for exogenous diacylglycerol. No metabolic effectors were detected by addition of heat treated extracts to the assay mixture. The above findings suggest that the higher enzyme activity at 20 degrees C can be attributed to a higher level of the enzyme rather than to the involvement of isozymes or metabolic effectors. Enzyme activity decreased rapidly during culture at 35 degrees C, indicating a rapid turnover of the enzyme. The level of temperature modulation was found to be a function of seed developmental stage.
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U. S. Department of Agriculture, Agricultural Research Service, Northern Regional Research Center, Seed Biosynthesis Research Unit, Peoria, Illinois 61604.
Cheesbrough
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