JBC, Vol. 250, Issue 9, 3552-3559, May, 1975
Carbamyl phosphate: glucose phosphotransferase and glucose-6-phosphate phosphohydrolase of nuclear membrane. Interrelationships between membrane integrity, enzymic latency, and catalytic behavior
H. M. Gunderson and R. C. Nordlie
The presence of carbamyl-phosphate:glucose phosphotransferase in liver
nuclei of five species of mammals and birds is demonstrated. The activity
is confined to nuclear membranes and is due exclusively to multifunctional
glucose-6-phosphatase-phosphotransferase (D-glucose-6-phosphate
phosphohydrolase; EC 3.1.3.9). The nuclear enzyme constitutes approximately
16 to 19 percent of total hepatic glucose-6-phosphatase-phosphotransferase.
Carbamyl-phosphate:glucose phosphotransferase and glucose-6-P
phosphohydrolase activities of membrane of chicken liver nuclei are shown
to be catalytically identical with the maximally activated microsomal
enzyme. A correspondence is seen in two-substrate kinetic double reciprocal
plots, K-m or apparent K-m values for the various substrates, K-i values
for the competitive inhibitors P-i and ATP, and pH-activity profiles.
Comparative studies were carried out with various intact, disrupted, and
detergent-dispersed membranous preparations by a combination of enzyme
kinetic and electron microscopic techniques. It is concluded that (a)
intimate interrelationships exists between catalytic behavior of this
enzyme and morphological integrity of membranes of which the enzyme is a
part; (b) activities of the enzyme of nuclear membrane appear quite
available for physiological phosphorylative functions; and (c)
interrelationships between membrane morphology and catalytic behavior of
this membrane-bound enzyme may well be involved in the bioregulation of
this complex, multifunctional enzyme system.
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