Biosynthesis of triterpenoid hydrocarbons in the B-race of the green alga Botryococcus braunii. Sites of production and nature of the methylating agent

Abstract

The B race of the green alga Botryococcus braunii is characterized by the production of large amounts of botryococcenes, i.e. triterpenoid hydrocarbons of general formula CπH2π-109 n= 30–37. The axenic strain used in this work produces botryococcenes ranging from C30 to C34 when fast growth is promoted by air-lift. Sequential extraction of hydrocarbons with solvents showed that botryococcenes accumulate in two distinct sites: externally in the successive outer walls forming a dense matrix and internally, probably in cyctoplasmic inclusions. Moreover, chase experiments after feeding the algae with sodium [1,2-14C]acetate, and feeding experiments with L-[Me-14C]methionine established the existence of an excretory process from the cells towards the matrix. The results of the radio GC analyses of the botryococcenes synthesized during the feeding experiments provided good evidence to show that the C30 botryococcene is the precursor of all the higher hydrocarbons, and that each intermediate botryococcene C31-C33 is the precursor of its next highest homologue. L-Methionine acts as the methyl donor in the methylation process, leading from the C30 precursor to the botryococcene family. The 13C NMR spectra of the botryococcenes produced when the algae were fed with L-[Me-13C]methionine indicate that the methylation takes place on the C30 backbone in positions 37, 16 and 20.