Histidase from the unicellular green alga Dunaliella tertiolecta: purification and partial characterization

In summer, the concentrations of dissolved organic nitrogen compounds are o ften higher than that of inorganic nitrogen. Under such conditions it would be advantageous if phytoplankton species could utilize organic nitrogen so urces, including free or combined amino acids, in addition to inorganic nit rogen. This study focused on histidine, the degradation of which potentiall y yields three nitrogen atoms for each molecule of histidine. In this work, histidase from Dunaliella tertiolecta, a deaminating enzyme catalysing the first steps of histidine degradation, was purified 4000-fold and partially characterized. The molecular weight of the native enzyme was estimated to be 155 kDa, corresponding to four subunits of 38 kDa. D. tertiolecta histid ase is stable in the presence of dithiothreitol and is inactivated by cyani de. Histidinol phosphate, histidine and Mn2+ are effective protectors again st cyanide inactivation. The enzyme did not exhibit classical Michaelis-Men ten kinetics but showed a relationship between the rate of catalysis (V) an d the concentration of substrate (S) that was characteristic of negative al losteric behaviour. A Hill coefficient of 4 was measured for histidine conc entrations higher than 22.5 mM. Guanine, xanthine and cytosine nucleotides are inhibitors of D. tertiolecta histidase.