Abstract: Stoichiometry of catalytic reaction by enzyme can be researched by scanning spectrophotometry. The difference absorption spectra at wave lengths between 200 and 400 nm were observed after starting the reaction at different time until no further change in the absorption profile could be detected.Therefore it was obtained that the absorption spectra of conversion of m hydroxybenzoate to protocatechuate by meta hydroxybenzoate 4-hydroxylase changed following the time. Concentration of each component was calculated from the molar extinction coefficient. The molar ratioes of substrate to product in two independent experiments were 1:0.998 and 1:1.085 respectively,indicating a good accord to the reaction by the MOB4 HO ase extracting from Co. testosteroni KH122-3S and Aspergillus niger genera.

Key words: meta hydroxybenzoate 4-hydroxylase, stoichiometry, absorption spectrum, Comamonas testosteroni 