UDPG:Sterol glucosyltransferase in etiolated pea seedlings

Sterol UDPglucose glucosyltransferase was located predominantly in the axis tissue of etiolated pea seedlings. During the first 11 days of growth the activity reached a peak in the axis tissue after seven days. Centrifugation of tissue homogenates showed the cell fraction sedimenting between 13000 nd 25000 g to have the highest specific activity and also the bulk of the total activity. Sitosterol is the major free sterol of this fraction and cholesterol is a trace component. The composition of the aglycones of the isolated steryl glycosides shows cholesterol to be the major sterol. Although exhibiting no metal ion requirement, the enzyme is stimulated by Ca²⁺ and Mg²⁺, partially inhibited by EDTA and EGTA and completely inhibited by Zn²⁺. The membranous nature of the enzyme is manifested by its stimulation by the addition of phosphatidyl -ethanolamine, -choline and -serine. After brief treatment with phospholipases A, C and D, enzyme activity is partially lost. After phospholipase A treatment the activity may be completely restored by the addition of phosphatidyl ethanolamine but phosphatidyl-choline and -serine are without effect. After phospholipase C and D treatment, each phospholipid brings about a partial recovery of activity but phosphatidyl ethanolamine is again superior.