Studies on the Biosynthesis and Metabolism of delta-Aminolevulinic Acid in Chlorella.
The regulation of chlorophyll synthesis in Chlorella was examined at the level of the formation and metabolism of delta-aminolevulinic acid. delta-Aminolevulinic acid synthetase activity could not be detected in broken cell preparations, and exogenously supplied delta-aminolevulinic acid was taken up only in the presence of dimethylsulfoxide, with a corresponding production of porphobilinogen.The delta-aminolevulinic acid dehydratase of Chlorella has a pH optimum of 7.8 and at this pH the Michaelis constant for delta-aminolevulinic acid is 0.13 millimolar.delta-Aminolevulinic acid excretion in the presence of levulinic acid, a competitive inhibitor of delta-aminolevulinic acid dehydratase, allowed measurement of the relative rates of delta-aminolevulinic acid synthesis under various growth conditions. A mutant which requires light for chlorophyll synthesis also requires light for delta-aminolevulinic acid accumulation in the presence of levulinic acid. This same mutant has 40% of the delta-aminolevulinic acid dehydratase activity of the wild-type Chlorella during growth in the dark on glucose.The necessity for protein synthesis during chlorophyll synthesis is due primarily to the requirement for protein synthesis during delta-aminolevulinic acid formation.It is concluded that the rate of chlorophyll formation and the cellular chlorophyll content are regulated by the relative rates of synthesis and breakdown of an enzyme responsible for delta-aminolevulinic acid biosynthesis and that this enzyme has an in vivo lifetime of about 30 minutes.
Beale
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SI
Department of Botanical Sciences, University of California, Los Angeles, California 90024.