The role of the glyoxylate cycle in algal metabolism.

Chlorella vulgaris, strain Brannon no.1 and Chlamydomonas reinhardi can convert acetate or ethanol into cell materials by the reactions of the glyoxylate cycle. In Chlorella this was found to operate during growth on these substrates but not during growth on glucose in the dark or on carbon dioxide in the light. This is reflected in the difference in the levels of malate synthase under these two growth conditions. However, the level of isocitrate lyase does not vary significantly according to the nature of the substrate, which is surprising, since in Chlamydomonas and most other organisms, glucose represses isocitrate lyase formation. The constitutive production of this enzyme in Chlorella cannot be explained by postulating a need to compensate for some enzymic deficiency in the tricarboxylic acid cycle or in routes leading to its replenishment, since these pathways were shown to operate quite normally in this strain. Partially purified isocitrate lyases from acetate- and glucose-grown cells were found to be so similar in charge, molecular weight, affinity for isocitrate and sensitivity to inhibition by phosphoenolpyruvate that they were considered to be identical molecules and not isoenzymes. However, the enzyme was found to be entirely soluble in glucose-grown cells, whereas in acetate-grown cells a large proportion of it was particle-bound. The excretion of glycollate by Brannon no.1 during growth on glucose in the dark provided evidence that the soluble enzyme is active in vivo even though it no longer has an anaplerotic function. The discovery of an organism unable to control the amount of isocitrate lyase formed but able to altar its spatial arrangement according to growth conditions suggests that there are two separate coarse control mechanisms associated with this enzyme, one controlling its level and the second determining its distribution.