ECTOAMINOPEPTIDASE SPECIFICITY AND REGULATION IN ANTARCTIC MARINE PELAGIC MICROBIAL COMMUNITIES

Ectoaminopeptidase activities of pelagic marine microbial communities were investigated on several research cruises in the western Antarctic Peninsula region from 1992 to 1994, using the fluorigenic substrate a nalogue L-leucyl-beta-naphthylamine. K-m values at in situ temperature were comparable to those observed by other investigators for a variet y of aquatic environments. Competitive inhibition by dipeptides of a v ariety of amino acids showed that the aminopeptidases present were bro adly specific; no strong tendency toward greater affinity for hydropho bic or hydrophilic amino acids was observed. Seawater cultures (1.0 mu m filtrate) were inoculated with a variety of monomeric organic compo unds and incubated for 24 to 72 h prior to activity determination; his tidine and phenylalanine were found to consistently inhibit aminopepti dase expression. It is hypothesized that auxotrophy for histidine and phenylalanine may be widespread in these assemblages, giving rise to h igh levels of constitutive, nonspecific aminopeptidase activity which results in significant respiration of the more common amino acids. Wha tever the exact mechanism, aminopeptidase activity is strongly affecte d by the specific compounds present and not simply by the carbon-to-ni trogen ratio.