Quantitative Studies of White Matter : I. Enzymes involved in glucose-6-phosphate metabolism

Total lipid and six enzymes closely related to the metabolism of glucose-6-phosphate have been measured in ten tracts of the rabbit. Lipid content appears to be a valid indicator of the degree of myelination. Heavily myelinated tracts have much larger amounts of glucose-6-phosphate dehydrogenase than lightly myelinated ones but there is no corresponding difference in 6-phosphogluconate dehydrogenase. In fact the ratios between the two enzymes were found to vary over a ninefold range. Hexokinase is found in largest amounts in tracts with relatively little lipid, and this tends to be true for phosphofructokinase as well. The fibrillar layer of olfactory bulb is exceptional with regard to both enzymes, and to glucose-6-phosphate dehydrogenase. The enzymes are present in amounts which are more than adequate to support glucose metabolism at a rate commensurate with the known rates of O(2) uptake by various tracts. The distribution of some of the enzymes is compatible with the notion that the nodes of Ranvier are regions of high metabolic activity. A simple algebraic relationship is found to hold fairly well for the distribution of four of the enzymes among the tracts.