Control of isocitrate lyase synthesis in Chlorella fusca var. vacuolata. The basal activity of the enzyme and the kinetics of induction

Effect of culture conditions on the isocitrate dehydrogenase and isocitrate lyase activities in Chlamydomonas reinhardtii

In the green alga Chlamydomonas reinhardtii, nitrogen staravation induced a reversible increase (2-fold) in NAD-isocitrate dehydrogenase (NAD-IDH; EC 1.1.1.41) and NADP-isocitrate dehydrogenase (NADP-IDH; EC 1.1.1.42) activities. Both enzymes were not affected by the concentration of CO₂ , the dark or the nature of the nitrogen source (nitrate, nitrite, or ammonium). When cells growing autotrophically were transferred to heterotrophic conditions, a 40% reduction of the NAD-IDH activity was detected, a 2-fold increase of NADP-IDH was observed and isocitrate lyase (ICL; EC 4.1.3.1) activity was induced. The replacement of autotrophic conditions led to the initial activity levels. NAD- and NADP-IDH activities showed markedly different patterns of increase in synchronous cultures of this alga obtained by 12 h light/12 h dark transitions. While NAD-IDH increased in the last 4 h of the dark period, NADP-IDH increased during the last 4 h of the light period, remaining constant for the rest of the cycle.

Isocitrate lyase of the alga Chlorogonium elongatum: Immunological quantification in relation to growth conditions

Isocitrate lyase (EC 4.1.3.1) from the acetate flagellate Chlorogonium elongatum was studied in relation to growth conditions. In cells growing heterotrophically or photoheterotrophically with acetate (2 g · l(-1)) specific activities of 0.453 and 0.335 μmol glyoxylate · min(-1) · mg(-1) protein, respectively, were found in the cell-free extracts. The malate synthase activity of heterotrophically grown Chlorogonium cells was found to be 0.572 nmol · min(-1) · mg(-1) protein. In the presence of inhibitors acting on transcription and translation there was no increase in isocitrate lyase activity; this would indicate that the enzyme was synthesized de novo in response to changing conditions in the surroundings. Using an enzyme immunoassay it was shown that the activity and amount of enzyme rose concomitantly when the cells were supplied with acetate, irrespective of whether they had been cultivated photoheterotrophically or heterotrophically on acetate.

Control of isocitrate lyase synthesis in Chlorella fusca var. vacuolata. Rate of enzyme synthesis in the presence and absence of acetate measured by [35S]methionine labelling and immunoprecipitation

The rate of increase of isocitrate lyase activity was measured in darkened Chlorella fusca var. vaculoata cultures in the presence and absence of acetate and compared with the rate of incorporation of [35S]methionine into isocitrate lyase enzyme protein under the same conditions. Isocitrate lyase enzyme protein was isolated for this purpose by specific immunoprecipitation and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. After 4h in the dark, in the presence of acetate the rate of increase of isocitrate lyase activity was 75 times that in the absence of acetate. Incorporation of [35S]methionine into isocitrate lyase was 140 times greater in the presence of acetate. Incorporation of [35S]methionine into the trichloroacetic acid-insoluble fraction overall was about five times as fast in the presence of acetate. These data are not consistent with an increased turnover of isocitrate lyase enzyme molecules, sufficient to account for the low rate of increase of isocitrate lyase activity in the absence of acetate. The greater rate of enzyme synthesis in the presence of acetate must therefore be due to some effect of this metabolite on the processing or translation of isocitrate lyase mRNA.