Purification and characterization of mannitol-l-phosphatase in the red alga Caloglossa continua (Ceramiales, Rhodophyta)

Purification of mannitol-1-phosphatase, an enzyme catalyzing the final step of mannitol biosynthesis, was first achieved in the mannitol-accumulating red alga Caloglossa continua (Okamura) King et Puttock. The enzyme was shown to be a monomer, since gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave close values of apparent molecular weights of 28,500 and 30,200, respectively. The protein exhibited an isoelectric point of 4.8. The substrate specificity for mannitol-1-phosphate (MIP) was very high, and that for K-m(MIP) was 0.41 mM. The catalytic activity was optimal at PH 7.4. The enzyme was activated by Mg2+, but was strongly inhibited by Ca2+, NaF, N-ethylmaleimide, and p-hydroxymercuribenzoic acid. Seawater levels of NaCl and physiological levels of mannitol also inhibited the activity by 50% or more. Changes in the concentrations of those ions and metabolites may regulate the biosynthesis of mannitol as an osmoregulant in vivo.