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The degradation of phenylalanine, tyrosine and related aromatic compounds by a marine diatom and a haptophycean alga Landymore, Arthur Frederick

Abstract

The degradation of phenylalanine and tyrosine was examined in axenic cultures of Isochrysis galbana Parke and Navicula incerta Hustedt. Both species were able to metabolize L-phenylalanine and L-tyrosine as the sole nitrogen source, but severe growth Inhibition was observed for I. galbana. No growth of I. galbana was obtained on the D-isomers of these two amino acids, but N. incerta was able to utilize both D-amino acids after an extended lag period. Analysis of the growth medium and the algal cells from non-radioactive and radioactive experiments never revealed cinnamic or p-coumaric acids. This suggested that phenylalanine and tyrosine ammonia-lyases (PAL and TAL) were not involved in the initial degradative step of either these amino acids. This was confirmed as no enzymatic activity for PAL was detected in crude enzyme preparations. Enzymatic activity for amino acid transaminase was obtained for both algal species. This suggested that phenylpyruvic acid and p-hydroxyphenylpyruvic acid were the Initial respective products from the metabolism of phenylalanine and tyrosine. From the non-radioactive and radioactive experiments, a scheme for the degradation of L-phenylalanine and L-tyrosine was proposed for both algal species. The compounds in brackets were not identified but were expected. The pathways were: L-phenylalanine → phenylpyruvic acid → [phenylacetaldehyde] → phenylacetic acid → mandelic acid → benzoylformic acid → [benzaldehyde] → benzoic acid → p-hydroxybenzoic acid; and L-tyrosine → p-hydroxyphenylpyruvic acid → [p-hydroxyphenyl acetaldehyde] → p-hydroxyphenylacetic acid → p-hydroxymandelic acid → [p-hydroxybenzoylformic acid] → p-hydroxybenzaldehyde → p-hydroxybenzoic acid and p-hydroxybenzyl-alcohol. Benzoic acid was also hydroxylated in the ortho and meta positions by both algal species. In both these schemes, the two C₁-fragments removed from the side chain were identified as CO₂ Also, the carbon-3 of the side chain of both phenylalanine and tyrosine was removed and trapped as CO₂. The relationship of these pathways to other algae is also discussed. Evidence suggested that p-hydroxybenzoic acid by both I. galbana and N. incerta was (1) decarboxylated, probably resulting in 1,4-dihydroxybenene, (2) brominated to 3-bromo-p-hydroxybenzoic acid, and (3) excreted into the medium. It was unknown if the 1,4-dihydroxybenene and/or 3-bromo-p-hydroxybenzoic acid caused the browning observed mainly in cultures of both species grown in the presence of tyrosine. Saltcyclic acid (ortho-hydroxybenzoic acid) was also decarboxylated, probably resulting in catechol (1,2-dihydroxybenzene). Ring cleavage observed for tyrosine and for phenylalanine (Vose et al.. 1971) appeared to involve a C₆-C₁ compound, probably benzoic acid or one of its hydroxylated products. Ring cleavage does not appear to be important in the degradation of either amino acid. No dihydroxyphenolic compounds were detected, but this does not eliminate the possibility of their formation. Evidence suggested that both species had problems hydroxylating not only the aromatic ring (eg. benzoic acid) but also the side chain of intermediates in the degradative pathway. Both algal species degraded cinnamic and p-coumaric acids in a similar pathway to that reported in higher plants. The pathway involved β-oxidation of the side chain to produce benzoic and p-hydroxybenzoic acids from cinnamic and p-coumaric acids respectively. The uptake rates of both phenylalanine and tyrosine and the effect of other aromatic compounds on the growth constants and lag periods of both algal species are also presented.

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