TETRAZOLIUM STUDIES OF THE RETINA: III. ACTIVITY OF METABOLIC INTERMEDIATES AND MISCELLANEOUS SUBSTRATES

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Abstract
Glucose intermediates induce a pattern of tetrazolium (T) reduction in the retina that may be likened to one or to both of the types of activity previously described as the lactate-diphosphopyridine nucleotide and the succinate types. The lactate-diphosphopyridine nucleotide type is produced by isocitrate-triphosphopyridine nucleotide and both appear to effect their reduction of T through the corresponding cofactor. The succinate type of activity is produced by pyruvate and [alpha]-ketoglutarate while both patterns of activity were found with glucose and malate substrates. The use of ethyl alcohol as a substrate led to equivocal results. D-Xylose, appeared to be metabolized by the retina and to induce a dual pattern of T precipitation, but L-xylose induced no activity. Fatty acids showed no dehydrogenase activity by the present means. Methylene blue induced a precipitation of T in the retina unlike that with any other substrate or cofactor. It is suggested that the methylene blue replaces diaphorase mediating the transfer of electrons from the endogenous dehydrogenases or cofactors to the T. These endogenous dehydrogenases and their substrates are relatively firmly bound to the tissue since they are not readily washed out of the retina, but they are used up by incubation of the retina with methylene blue prior to addition of T. The limitations of the T technique are discussed with particular reference to their lack of specificity in indicating sites of specific dehydrogenases.