Estrogen Modification of Human Glutamate Dehydrogenases Is Linked to Enzyme Activation State

Abstract

Mammalian glutamate dehydrogenase (GDH) is a housekeeping enzyme central to the metabolism of glutamate. Its activity is potently inhibited by GTP (IC 50 ‫؍‬ 0.1-0.3 M) and thought to be controlled by the need of the cell in ATP. Estrogens are also known to inhibit mammalian GDH, but at relatively high concentrations. Because, in addition to this housekeeping human (h) GDH1, humans have acquired via a duplication event an hGDH2 isoform expressed in human cortical astrocytes, we tested here the interaction of estrogens with the two human isoenzymes. The results showed that, under base-line conditions, diethylstilbestrol potently inhibited hGDH2 (IC 50 ‫؍‬ 0.08 ؎ 0.01 M) and with ϳ18-fold lower affinity hGDH1 (IC 50 ‫؍‬ 1.67 ؎ 0.06 M; p < 0.001). Similarly, 17␤-estradiol showed a ϳ18-fold higher affinity for hGDH2 (IC 50 ‫؍‬ 1.53 ؎ 0.24 M) than for hGDH1 (IC 50 ‫؍‬ 26.94 ؎ 1.07 M; p < 0.001). Also, estriol and progesterone were more potent inhibitors of hGDH2 than hGDH1. Structure/function analyses revealed that the evolutionary R443S substitution, which confers low basal activity, was largely responsible for sensitivity of hGDH2 to estrogens. Inhibition of both human GDHs by estrogens was inversely related to their state of activation induced by ADP, with the slope of this correlation being steeper for hGDH2 than for hGDH1. Also, the study of hGDH1 and hGDH2 mutants displaying different states of activation revealed that the affinity of estrogen for these enzymes correlated inversely (R ‫؍‬ 0.99; p ‫؍‬ 0.0001) with basal catalytic activity. Because astrocytes are known to synthesize estrogens, these hormones, by interacting potently with hGDH2 in its closed state, may contribute to regulation of glutamate metabolism in brain.

The Glutamate Dehydrogenase Pathway and Its Roles in Cell and Tissue Biology in Health and Disease

“…As noted above, steroid hormones interact with hGDH2 with a much greater affinity than with hGDH1 [51,56,57]. Thus, inhibitory assays, performed under baseline conditions, revealed that diethylstilbestrol (DES) (IC 50 = 0.08 ± 0.01 μM) and 17 beta-estradiol (IC 50 = 1.67 ± 0.06 μM) inhibit hGDH2 with ~ 18-fold higher affinity than hGDH1 (IC 50 = 1.53 ± 0.24 μM for DES and IC 50 = 26.94 ± 1.07 μM; p < 0.001 for 17 beta-estradiol) [56].…”

Multiple Forms of Glutamate Dehydrogenase in Animals: Structural Determinants and Physiological Implications

“…hGDH2 is about 20-fold more sensitive to one of the female sex hormones, 17β-estradiol (IC50 = 1.53 μM), than is hGDH1 (IC50 = 26.94 μM) [106]. It was observed that female patients with mutation Ser445Ala in the GTP-binding site of hGDH2 are somewhat protected from the early development of Parkinson disease (see Section 8.2) than are male patients.…”

The Role of Glutamate Dehydrogenase in Mammalian Ammonia Metabolism

“…Hill plot analyses showed that GTP binding to hGDH2 was not co-operative as is the case for hGDH1 (Kanavouras et al 2007). In addition, hGDH2 is markedly sensitive to estrogens and to neuroleptic drugs (Borompokas et al 2010;Plaitakis et al 2011). …”

“…Of these, substitution of Gly456 for Ala dissociated hGDH1 from GTP control, whereas replacement of Arg443 by Ser diminished basal activity while permitting full activation by ADP/L-leucine. In addition, the Arg443Ser substitution lowered the optimal pH and rendered the enzyme heat-labile and markedly sensitive to inhibition by estrogens and neuroleptic agents (Borompokas et al 2010;Plaitakis et al 2011).…”