Asparagine catabolism in bryophytes: Purification and characterization of two L‐asparaginase isoforms from Sphagnum fallax

Abstract

Nitrogen represents a critical nutrient in raised bogs. In Sphagna, dominating this habitat, the prevalent storage amino acid asparagine is catabolized predominantly by the enzyme L‐asparaginase (EC 3.5.1.1). L‐asparaginase activity has been detected in each of 10 Sphagnum species investigated. In Sphagnum fallax Klinggr. (Klinggr. clone 1) cultivated under axenie conditions in continuous feed bioreactors, the enzyme displayed a light dependent increase in activity. We separated two isoforms, designated L‐asparaginase 1 and 2, characterized by their different elution patterns from an anion‐exchange column. In stem segments only L‐asparaginase 2 could be detected, whereas in capitulae L‐asparaginase 1 represented the dominating isoform. Purified chloroplasts displayed no L‐asparaginase activity. Almost the entire activity was located in the cytosohc fraction. L‐asparaginase 1 and 2 have been purified 82‐fold and 188‐fold, respectively, by ion‐exchange, size‐exclusion and hydrophobic interaction chrornatography. Identical pH optima (8.2) and molecular weights (126 000) were determined. The Km values for asparagine (7.4 mM for L‐asparaginase 1 and 6.2 mM for L‐asparaginase 2) were in the range of those described for higher plants. On the other hand Sphagnum L‐asparaginase is comprised of four subunits as are the L‐asparaginases of microorganisms. So, the characteristics of the bryophyte enzyme appear to be intermediate between those from higher plants and those from microorganisms.