NADPH DIAPHORASE IS NOT INHIBITED BY ETHYLENEDIAMINETETRAACETIC ACID AND IS NOT SPECIFIC FOR NITRIC-OXIDE SYNTHASE IN THE CHOROID-PLEXUS OF RAT AND MOUSE

NADPH diaphorase is a bistochemical activity which, in formaldehyde-fixed tissue, is rather specific for nitric oxide synthase. Recently, it was shown that NADPH diaphorase activity is inhibited by ethylenediaminetetraacetic acid (EDTA) in neurons but not in the choroid plexus epithelium The present study, while confirming these results, demonstrates that the apparent sensitivity of NADPH diaphorase for EDTA reflects only the dependence of malic enzyme, which is used as the source of reduced cofactor, on Mg2+ or Mn2+ ions. Furthermore, evidence is provided that the apparent EDTA-insensitive NADPH diaphorase activity in the choroid plexus reflects the activity of alkaline phosphatase in conjunction with NADH diaphorase. Apart from these pitfalls, the use of the indirect, malic enzyme based method for NADPH diaphorase was found to cause much higher background staining compared to the direct method using NADPH, and is therefore proposed to be abandoned.