Kinetic characterization of adenosine deaminase activity in zebrafish (Danio rerio) brain

Adenosine deaminase (ADA; EC 3.5.4.4) activity is responsible for cleaving adenosine to inosine. In this study we described the biochemical properties of adenosine deamination in soluble and membrane fractions of zebrafish (Danio rerio) brain. The optimum pH for ADA activity was in the range of 6.0-7.0 in soluble fraction and reached 5.0 in brain membranes. A decrease of 31.3% on adenosine deamination in membranes was observed in the presence of 5 mM Zn(2+), which was prevented by 5 mM EDTA. The apparent K(m) values for adenosine deamination were 0.22+/-0.03 and 0.19+/-0.04 mM for soluble and membrane fractions, respectively. The apparent V(max) value for soluble ADA activity was 12.3+/-0.73 nmol NH(3) min(-1) mg(-1) of protein whereas V(max) value in brain membranes was 17.5+/-0.51 nmol NH(3) min(-1) mg(-1) of protein. Adenosine and 2'-deoxyadenosine were deaminated in higher rates when compared to guanine nucleosides in both fractions. Furthermore, a significant inhibition on adenosine deamination in both soluble and membrane fractions was observed in the presence of 0.1 mM of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA). The presence of ADA activity in zebrafish brain may be important to regulate the adenosine/inosine levels in the CNS of this species.