Molecular and biochemical characterization of nitric oxide synthase isoforms and their intracellular distribution in human peripheral blood mononuclear cells.

Nitric oxide synthase (NOS) expression and catalytic status in human peripheral blood mononuclear cells (PBMCs) is debatable, while its sub-cellular distribution remains unascertained. The present study characterizes NOS transcripts by real time PCR, NOS protein by immunoprecipitation (IP)/Western blot (WB), nitric oxide (NO) generation by DAF-2DA and NOS sub-cellular distribution by immunogold electron microscopy in resting PBMCs, monocytes and lymphocytes obtained from healthy donors. We observed constitutive expression of full length NOS isoforms (nNOS, iNOS and eNOS) in PBMCs: with the highest expression of iNOS in comparison to nNOS and eNOS. Isolated monocytes expressed more eNOS transcript and protein as compared to nNOS and iNOS. Lymphocytes however had more iNOS transcripts and protein than nNOS and eNOS. NOS was catalytically active in PBMCs, monocytes as well as in lymphocytes as evident by NO generation in the presence of substrate and cofactors, which was significantly reduced in the presence of NOS inhibitor. Immunogold electron microscopy and morphometric analysis revealed the distinct pattern of NOS distribution in monocytes and lymphocytes and also exhibited differences in the nuclear-cytoplasmic ratio. nNOS localization was much more in the cytosol than in the nucleus among both monocytes and lymphocytes. Interestingly, iNOS distribution was comparable in both cytosol and nucleus among monocytes, but in lymphocytes iNOS was predominantly localized to the cytosol. The present study exhibits constitutive presence of all the NOS isoforms in PBMCs and reports the distinct pattern of NOS distribution among monocytes and lymphocytes.