Three tetrazolium salts are used as histochemical indicators of dehydrogenase activity in the skin of sheep and other mammals. Of these compounds 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl-tetrazolium chloride (INT) is shown to be the most satisfactory. It enables various histological structures of the wool follicle to be conveniently demonstrated.

In the presence of appropriate substrates and DPN and ATP, the intracellular deposition of the granular formazan in frozen sections has revealed the distribution of succinic, α-glycerophosphoric, lactic, and malic dehydrogenases, and regions where some phosphorylated carbohydrate intermediates promote reduction. Metabolic activity in the Malpighian layers and in the undifferentiated epidermal cells of the hair matrix reduces INT enzymatically up to the pre-keratinous zone. Here the protein-bound sulphydryl groups reduce INT and produce intense red staining which is easily distinguished from the granular enzymic staining.

The distribution of cytochrome oxidase in the wool follicle resembles the distribution of dehydrogenase activity.

INT is used for the estimation of succinic dehydrogenase in skin and kidney tissues.

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