Light Modulation of Glucose-6-Phosphate Dehydrogenase

Abstract

Light inactivation of glucose-6-phosphate dehydrogenase within the pea (Pisum sadvum L.) leaf chloroplast has a narrow pH optimum between 7.2 and 7.4 and is NADP-sensitive. The pH optimum for dark activation is slightly lower. Inactivation apparently results in a simple decrease in maximal velocity of the chloroplastic and cytoplasmic forms of the enzyme with no concomitant change in pH optimum or Km (glucose 6-phosphate).The chloroplastic form of glucose-6-P dehydrogenase (EC 1.1.1.49) is inactivated by a dithiol-containing light effect mediator which is associated with the photosynthetic electron transport system (3) and inhibited by NADPH (13,20). The purpose of the present experiments was to examine the role of these and other types of modulation in the control of the activity of this enzyme and hence of the potential operation of the oxidative pentose phosphate pathway within the chloroplast.MATERIALS AND METHODS Plant Material. Pea (Pisum sativum L., var. Little Marvel) plants were grown in a soil-perlite mixture under natural light for 12 to 15 days in a greenhouse.Preparation of Extracts and Chloroplasts. Whole shoot extracts were prepared by grinding green shoots in 20 mm, pH 7.4, tris-HCl with a mortar and pestle, filtering through four layers of cheesecloth, and centrifuging for 15 min at 12,000g. Chloroplasts and cytoplasmic fractions were prepared essentially as described previously (2) except that in some cases (specified) the isotonic medium of Stokes and Walker (18) was used. Stromal and particulate fractions of broken chloroplasts were prepared as described previously (3) except that for determination of pH optima of light inactivation and dark activation the concentration of HEPES in the resuspension medium was only 5 mm and in all cases MgC12 was only 2 mm. Enzyme Assays. Glucose-6-P dehydrogenase and NADPlinked glyceraldehyde-3-P dehydrogenase in NADP-generating direction were assayed as previously described (7).Determination of pH Optima. Assay mixture pH was mea-' This work was supported by National Science Foundation Grant BMS 75-02281.2To whom reprint requests should be addressed.sured at room temperature with a Radiometer pH meter 26 after activity of the enzyme or system was determined.Determination of Kinetic Constants. The activity of a constant amount of the specified enzyme was measured at six different concentrations of substrate. Each set of reaction rates and corresponding substrate concentrations was analyzed using the program of Hanson et al. (9) and the IBM 370 computer at the

Nitrogen Assimilation, Abiotic Stress and Glucose 6-Phosphate Dehydrogenase: The Full Circle of Reductants

“…The presence of a G6PDH activity confined in chloroplasts has been widely demonstrated [ 15 , 16 ]: this chloroplastic enzyme, generally defined as P1-G6PDH, is reversibly inhibited by light to guarantee an efficient photosynthesis, and avoid a futile cycle; in the dark this inhibition is removed, and the OPPP is activated to produce reducing equivalents [ 16 ].…”

Glucose‐6‐Phosphate Dehydrogenases

“…Anderson et al have examined the mechanism of light inactivation of the G6PDs from pea leaves (9)(10)(11)(12) and the blue-green alga Anacystis niduluns (1 14). They found that dithiothreitol simulates the light effect (9,10,-12,114) and provided evidence suggesting that, in vivo, the inactivation of GGPD is mediated by way of the photochemical apparatus, possibly involving a thylakoid membrane-bound vicinal dithiol (9).…”

Stress responses in alfalfa (Medicago sativa L.) XIX. Transcriptional activation of oxidative pentose phosphate pathway genes at the onset of the isoflavonoid phytoalexin response

“…The activities of the two forms of both enzymes are regulated by NADPH:NADP ratio, as NADPH is a potent competitive inhibitor with respect to NADP [11,23]. Both forms of G6PDH are inactivated by light via a redox-regulated reaction [2]. Phenylpropanoid pathway intermediates have been shown to inhibit both tobacco 6PGDH isoenzymes [ 1 ], although the physiological significance of this is not clear.…”