, Volume 15, Issue 3, pp 201-212

Glutamate dehydrogenases from chlorella: Forms, regulation and properties

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Summary

Despite the great physiological importance of glutamate dehydrogenases (GDHs) fromChlorella, practically no pertinent data were available until 1965. We made an attempt to fill the gap and to determine the forms of GDH present inChlorella cells, the dependence of their synthesis on the nitrogen source, their physico-chemical and kinetic properties and their functional features in the cell. The thermophilic strainChlorella pyrenoidosa Pringsheim 82 T was selected for this purpose because it was shown to have both NAD- and NADP-GDH activities, the NADP-GDH activity being much higher under conditions of NH 4 + assimilation than in NO 3 - assimilation. Our findings allow the following conclusions to be made:

  1. The genome ofCh.pyrenoidosa Pr. 82 T contains information required for the synthesis of three GDHs depending on the environmental conditions. One GDH which is active with both cofactors NAD(P)-GDH is synthesized in the constitutive way, another, NADP-specific GDH, is synthesizedde novo under the influence of NH 4 + , and the third GDH, also NADP-specific, is synthesized during NH 4 + assimilation only in cells rich in glycolytic products.

  2. Constitutive and NH 4 + -inducible GDHs were separated and obtained in homogeneous and highly purified states, respectively.

  3. Constitutive GDH is a hexamer of 6 × 49,000± 1,000 with a rigid stable structure.

    Inducible GHD is an allosteric enzyme with a very labile structure. Its allosteric activator is NADPH. This enzyme is characterized by slow conformational transformations (isomerization) that accompany transition from inactive (low activity) to active state. SH-groups play a major role in this change.

  4. Constitutive GDH is characterized by unidirectional inhibition by p-chloro-mercuribenzoate (PCMB).

  5. Inducible GDHin vivo functions mainly in the direction of synthesis of glutamate.

    Possible localization and pattern of action of constitutive GDH are discussed.

A submitted article.