Direct effect of temperature on the catalytic activity of nitric oxide synthases types I, II, and III.
The effect of temperature (between 5.0 and 45.0 degrees 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 V(max) for NOS-I activity increases from 1.8 x 10(1) pmol min(-1) mg(-1), at 5.0 degrees C, to 1.8 x 10(2) pmol min(-1) mg(-1), at 45.0 degrees C; on the other hand, the value of K(m) (=4.0 x 10(-6) M) is temperature independent. Again, the value of V(max) for NOS-II activity increases from 8.0 pmol min(-1) mg(-1), at 7.0 degrees C, to 5.4 x 10(1) pmol min(-1) mg(-1), at 40.0 degrees C, the value of K(m) (=1.8 x 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 DeltaH(V(max)) (=4.2 x 10(1) kJ mol(-1)), DeltaH(K(m)) (=0 kJ mol(-1)), and DeltaH((V(max))(/K(m))()) (=4.2 x 10(1) kJ mol(-1)). On the contrary, the value of K(m) for NOS-III activity decreases from 3.8 x 10(-5) M, at 10.0 degrees C, to 1.6 x 10(-5) M, at 40.0 degrees C, the value of V(max) (=6.8 x 10(1) 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 K(m) being temperature independent. However, when l-arginine level is higher than 2 x 10(-4) M, as observed under in vivo conditions, NOS-III activity is essentially unaffected by temperature, the substrate concentration exceeding the value of K(m). As a whole, although further studies in vivo are needed, these observations seem to have potential physiopathologic implications.
Venturini
G
G
Department of Biology, University of Rome Tre, Viale Guglielmo Marconi 446, Rome, I-00146, Italy. venturin@bio.uniroma3.it
Colasanti
M
M
Fioravanti
E
E
Bianchini
A
A
Ascenzi
P
P
D000818:Animals; D002384:Catalysis; D002417:Cattle; D002478:Cells, Cultured; D004730:Endothelium, Vascular / Q000201:enzymology; D019001:Nitric Oxide Synthase / Q000378:metabolism*; D052248:Nitric Oxide Synthase Type I; D052247:Nitric Oxide Synthase Type II; D052250:Nitric Oxide Synthase Type III; D013696:Temperature
D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; D052247:Nitric Oxide Synthase Type II; D052250:Nitric Oxide Synthase Type III