LIMITATIONS IN THE HISTOCHEMICAL DEMONSTRATION OF PYRIDINE NUCLEOTIDE-LINKED DEHYDROGENASES ("NOTHING DEHYDROGENASE")

1. The ability of SH groups in well-dialyzed protein fractions to reduce exogenous pyridine nucleotides has been described in the biochemical literature. 2. This effect has been shown to be the cause of the positive histochemical result obtained when fresh tissue sections are incubated at alkaline pH levels with oxidized pyridine nucleotides (DPN or TPN) and tetrazolium salts, in the absence of any substrate. 3. The effect is pH-dependent, commencing at about pH 7.0 and rising to a maximum between pH 8.0 and 9.0. It is abolished by low concentrations of sulphydryl inhibitors or by heating the tissues. 4. The specific diaphorases are unaffected by sulphydryl inhibitors and these are presumed to transfer hydrogen and electrons from the reduced pyridine nucleotides (DPNH, TPNH) to the tetrazolium salt. 5. The development of histochemical methods for specific pyridine nucleotide-linked dehydrogenases is hindered by the activity described, to which Racker's convenient term "nothing dehydrogenase" has been applied.