Overexpression of neuronal nitric oxide synthase in insect cells reveals requirement of haem for tetrahydrobiopterin binding.

Nitric oxide synthase (NOS) catalyses the conversion of L-arginine into L-citrulline and nitric oxide. Recently we have developed a method for expression of recombinant rat brain NOS in baculovirus-infected Sf9 cells and purification of the enzymically active enzyme [Harteneck, Klatt, Schmidt and Mayer (1994) Biochem J. 304, 683-686]. To study how biosynthetic manipulation of the NOS cofactors haem, FAD/FMN, and tetrahydrobiopterin (H4biopterin) affects the properties of the isolated enzyme, Sf9 cells were infected in the absence and presence of haemin chloride (4 microg/ml), riboflavin (0.1.mM), and the inhibitor of H4biopterin biosynthesis 2,4-diamino-6-hydroxypyrimidine (10 mM). In the absence of haemin, NOS was expressed to a very high level but remained predominantly insoluble. Purification of the soluble fraction of the expressed protein showed that it had poor activity (0.35 micromol of citrulline x mg(-1) x min(-1)) and was haem-deficient (0.37 equiv. per monomer). Supplementing the culture medium with haemin resulted in pronounced solubilization of the expressed enzyme, which had a specific activity of approximately 1 micromol of citrulline x mg(-1) x min(-1) and contained 0.95 equiv. of haem per monomer under these conditions. Unexpectedly, the amount of H(4) biopterin endogenously present in the different NOS preparations positively correlated with the amount of enzyme-bound haem (y = 0.066+0.430x; r = 0.998). Radioligand binding experiments demonstrated that haem-deficient enzyme preparations containing 30-40% of the holoenzyme bound only approximately 40% of H4biopterin as compared with haem-saturated controls. These results suggest that the prosthetic haem group is essentially involved in the correct folding of NOS that is a requisite for solubilization of the protein and tight binding of H4biopterin.