Molecular inactivation of testicular hyaluronidase in solid state after proton irradiation: a study based on target size, substrate binding and thermodynamic analysis of heat denaturation.

Dose response activity curve of testicular hyaluronidase (HDase) following proton irradiation in dry state follows complicated mechanisms which may involve multiple hits and multiple targets of variable sizes giving a constant G value of 1.66. Target analysis appears to be modified by slow recovery of activity when irradiated enzyme is brought to aqueous phase. However, pattern of irradiation at a dose of 1 x 10(5) to 8 x 10(5) Gy reveals that though binding affinity of enzyme to the substrate (hyaluronic acid) increases as shown by declining Km from 500 mg/l to 300-70 mg/l, the reaction rate of catalysis by irradiated HDase is decreased due to decrease in reaction velocity (Vmax: 266 versus 76 units at 8 x 10(5) Gy). Activation analysis of heat denaturation of nonirradiated HDase suggested the involvement of 78 kcal/mole of energy of activation (E*a) which declined to 63-52 k cal/mole after irradiation at 1 x 10(5) to 8 x 10(5) Gy for residual enzyme. The corresponding change in entropy of activation (delta S*) increased from a control value of -291 eu to -236 eu at 8 x 10(5) Gy. From thermodynamic analysis in association with recovery in aqueous phase, it is concluded that HDase is inactivated due to dissipation of proton energy among weak forces including H bonds associated with secondary/tertiary structure of molecules.