Linearity in dehydrogenase reaction rate studies in tissue sections is affected by loss of endogenous substrates during the reaction.

We studied the effect of section thickness on the reaction rate of glucose-6-phosphate dehydrogenase (G6PD) activity in unfixed sections of rat liver by use of continuous monitoring by microdensitometry of the reaction product as it formed in the section during incubation. Tetranitro BT or nitro BT was used as final electron acceptor and polyvinyl alcohol as tissue stabilizer. Each test minus control reaction curve deviated from linearity during the first 2 min of incubation. This was mainly due to loss of low molecular weight endogenous dehydrogenase substrates from the surface of the section. For any given reaction, the same absolute amount of endogenous substrate was lost from each section, and hence a much greater proportion was lost from the thinner sections. Such losses lead to a deficit in (nonspecific) formazan production. There was a greater loss from, and hence a greater deficit in, formazan production in sections incubated at 30 degrees C than at 37 degrees C and when nitro BT was used instead of tetranitro BT, but the greatest loss of endogenous substrates occurred in sections incubated in control media. Therefore, greater losses seemed to occur when the reactions were slower because of failure to overcome the critical supersaturation level of the formazan. A consequence of this was a non-linear test minus control response during the first minutes of the incubation.