Direct Effect of Temperature on the Catalytic Activity of Nitric Oxide Synthases Types I, II, and III

Abstract

The effect of temperature (between 5.0 and 45.0°C) on the catalytic activity of nitric oxide synthases types I, II, and III (NOS-I, NOS-II, and NOS-III, respectively) has been investigated, at pH 7.5. The value of Vmax for NOS-I activity increases from 1.8 × 101 pmol min−1 mg−1, at 5.0°C, to 1.8 × 102 pmol min−1 mg−1, at 45.0°C; on the other hand, the value of Km (=4.0 × 10−6 M) is temperature independent. Again, the value of Vmax for NOS-II activity increases from 8.0 pmol min−1 mg−1, at 7.0°C, to 5.4 × 101 pmol min−1 mg−1, at 40.0°C, the value of Km (=1.8 × 10−5 M) being unaffected by temperature. Temperature exerts the same effect on NOS-I and NOS-II activity, as shown by the same values of ΔHVmax (=4.2 × 101 kJ mol−1), ΔHKm (=0 kJ mol−1), and ΔH(Vmax/Km) (=4.2 × 101 kJ mol−1). On the contrary, the value of Km for NOS-III activity decreases from 3.8 × 10−5 M, at 10.0°C, to 1.6 × 10−5 M, at 40.0°C, the value of Vmax (=6.8 × 101 pmol min−1 mg−1) being temperature independent. Present results indicate that temperature influences directly NOS-I and NOS-II activity independently of the substrate concentration, the values of Km being temperature independent. However, when l-arginine level is higher than 2 × 10−4 M, as observed under in vivo conditions, NOS-III activity is essentially unaffected by temperature, the substrate concentration exceeding the value of Km. As a whole, although further studies in vivo are needed, these observations seem to have potential physiopathologic implications.