L-Threonine deaminase in marine planktonic algae

Abstract

1. The “biosynthetic” l-threonine (deaminating) dehydratase activity of 7 marine planktonic species from 5 classes of algae showed high substrate specificity toward l-threonine, with the exception of one alga. The algal extracts deaminated l-serine and l-allothreonine at rates which were 20–25 and 5–10%, respectively, of that of l-threonine, and these reaction were inhibited by l-isoleucine. d-Threonine, d-serine, l-homoserine, and l-O-methylthreonine were ineffective as substrates. 2. Extracts of the cryptophyte Chroomonas salina were exceptional in showing about twice the activity with l-serine relative to l-threonine. The reaction with l-serine was insensitive to inhibition by l-isoleucine and differed, in several respects from that with l-threonine. It was inferred that this algal extract contains a specific l-serine deaminase in addition to the regular l-threonine deaminase. 3. The rate of deamination of l-threonine by the algal extracts was not affected by the simultaneous presence of l-allothreonine but was markedly inhibited by l-serine and l-homoserine. 4. Kinetic studies of the effects of graded concentrations of the substrate analogs at a fixed (saturating) substrate level or of substrate concentration at fixed analog levels indicated that l-serine inhibited the threonine deamination by irreversible competition with the substrate, whilst l-homoserine affected this reaction by an unknown (presumably allosteric) mechanism not involving substrate competition.