A Phosphate Activated Glutaminase in Rat Liver Different from that in Kidney and other Tissues

Abstract

Summary. The hydrolysis of glutamine by rat liver preparations occurred through a phosphate-dependent reaction differing from the one in rat kidney preparations primarily in its greater affinity for phosphate. The two enzymes also differed in pH optima, affinity for glutamine, reactions with various activators and inhibitors, and distributions in tissues. Both enzymes were mitochondrial, but retained their distinctive differences after sonication and in mixtures. The conditions for the independent assay of each type of activity in tissues are described. The liver type of reaction was limited to adult and regenerating liver, while high activities of the kidney-type of reaction were found in brain, kidney and small intestine and lesser activities in a variety of other tissues, including fetal liver and hepatoma. A third type of activity occurred independently of phosphate addition and comprised onesixth of the total in kidney preparations and less in other tissues.

Glutamine metabolism in the rat

“…This basal activity is increased about 20-fold by the presence of HCO, NH: and ATP [ 13, 151. In surveying the literature on glutaminases it is very difficult to obtain a consistent picture. The 'phosphate-dependent' glutaminases of liver, kidney and brain described by Katunuma et al [ 191 are all different, and the liver enzyme, in regard to heat inactivation, pH optimum and inhibition bymercurials, does not correspond with the liver enzyme described by Horowitz and Knox [2]. Recently, on the basis of inhibition by p-mercuribenzoate, two distinct phosphate-dependent mitochondrial glutaminases are reported to be present in both pig kidney cortex and rat brain [20], again contrary to the findings of Horowitz and Knox [2].…”

“…The mitochondrial 'phosphate-dependent' isoenzymes are considered to be the true glutaminases. Liver, and possibly lung, possess the 'liver-type' isoenzyme; all other tissues, including brain and lung, have the 'kidney-type' [2]. The two are distinguishable on the basis of Pi requirement (the 'liver-type' requires a low concentration for activation, the 'kidney-type' a high concentration), pH optima, affinity for glutamine, reactions with activators and inhibitors [2] and inhibition by glutamate [I].…”

“…The two are distinguishable on the basis of Pi requirement (the 'liver-type' requires a low concentration for activation, the 'kidney-type' a high concentration), pH optima, affinity for glutamine, reactions with activators and inhibitors [2] and inhibition by glutamate [I]. The tissue distribution of glutaminase activity reported by different workers is not identical, but highest activity is present in kidney, brain and possibly small intestine, with much lower activity in liver and other tissues [2,3].…”

“…Its activity parallels the amount of haematopoietic, not the parenchymal, tissue. Although the phosphate requirement of the liver enzyme is low (Km 2.8 mM) the Km for glutamine is very high: 28 mM [5] to 42 mM [2]. Observations that low concentrations of glutamine are not metabolized by perfused liver [6] or hepatocytes [7] led to the suggestion that the liver enzyme does not degrade glutaDedicated to Professor Sir Hans Krebs, FRS, on his eightieth blrthday K86 mine under physiological conditions [7].…”

“…None of these activators has been tested with the solubilized enzyme. It is also worth remembering that the activity in liver [2,3] was determined before the allosteric nature of glutaminase was known. This basal activity is increased about 20-fold by the presence of HCO, NH: and ATP [ 13, 151. In surveying the literature on glutaminases it is very difficult to obtain a consistent picture.…”