It was found that methylene blue caused inhibition of enzyme and that its kinetic properties of sheep mammary tissue arginase in lactation. Arginase activity was spectrophotometrically measured using the Tiosemicarbazide diacetylmonoxime urea (TDMU) method.

Following preincubation with MnCl2, exposure to a 150 W (watt) light source and the addition of methylene blue, the activity of arginase was found to be lower than that after preincubation with a 150 W light source, methylene blue and the addition of MnCl₂.

Enzyme inhibition was reduced by 76% in daylight, 63% in darkness and 66% in 150 W light source. The inhibitory effect of methylene blue on sheep mammary tissue arginase was studied and inhibition was found to be noncompetitive.

We concluded that photoinactivation might probably show its effect by modifying imidazole grups of histidyl residues in arginase molecules.