Szweda L I, Atkinson D E
Department of Chemistry and Biochemistry, University of California, Los Angeles 90024.
Biochim Biophys Acta. 1990 Nov 15;1041(2):201-6. doi: 10.1016/0167-4838(90)90066-o.
The activity of rat liver glutaminase from sedimented fractions of freeze-thawed mitochondria is strongly affected by variation in the Mg2+ concentration within the approximate physiological range of activators. A rise in the Mg2+ concentration stimulates glutaminase by increasing the apparent affinity of the enzyme for its positive modifier phosphate. With the addition of 4 mM Mg2+ the M0.5 for phosphate activation decreased from 18 to 9.5 mM at pH 7.1, 10 to 5.8 mM at pH 7.4 and 6.4 to 4.0 mM at pH 7.7. The result is an increase in the apparent affinity of the enzyme for glutamine. With the addition of 4 mM Mg2+ the S0.5 of glutaminase for glutamine decreased from 24 to 13 mM at pH 7.1, 14 to 9.6 mM at pH 7.4, and remained unchanged at 8.2 mM at pH 7.7. Since Mg2+ stimulates glutaminase, as does a rise in pH (Szweda, L.I. and Atkinson, D.E. (1989) J. Biol. Chem. 264, 15357-15360), by increasing the apparent affinity of the enzyme for phosphate, it reduces the inhibitory effect of a decrease in pH and/or phosphate concentration over a physiologically relevant range.
冻融线粒体沉淀组分中的大鼠肝脏谷氨酰胺酶活性，在激活剂的近似生理浓度范围内，会受到Mg2+浓度变化的强烈影响。Mg2+浓度升高会通过增加该酶对其正性修饰剂磷酸盐的表观亲和力来刺激谷氨酰胺酶。添加4 mM Mg2+后，在pH 7.1时，磷酸盐激活的M0.5从18 mM降至9.5 mM；在pH 7.4时，从10 mM降至5.8 mM；在pH 7.7时，从6.4 mM降至4.0 mM。结果是该酶对谷氨酰胺的表观亲和力增加。添加4 mM Mg2+后，在pH 7.1时，谷氨酰胺酶对谷氨酰胺的S0.5从24 mM降至13 mM；在pH 7.4时，从14 mM降至9.6 mM；在pH 7.7时，保持在8.2 mM不变。由于Mg2+通过增加该酶对磷酸盐的表观亲和力来刺激谷氨酰胺酶，这与pH升高的作用相同（Szweda, L.I.和Atkinson, D.E. (1989) J. Biol. Chem. 264, 15357 - 15360），所以在生理相关范围内，它会降低pH降低和/或磷酸盐浓度降低的抑制作用。
