Absence of the glycerol phosphate shuttle in the various zones of the growth plate.
UNLABELLED: The various zones of the growth plate of the rib of six-week-old male New Zealand White rabbits were obtained by means of a specially devised guillotine slicing apparatus. Cartilage slices from each zone were assayed enzymatically for activity of glycerol phosphate dehydrogenase by an adaptation of a fluorimetric technique based on the conversion of non-fluorescent resazurin to the highly fluorescent resorufin in the presence of nicotinamide adenine dinucleotide and the enzyme. No glycerol phosphate dehydrogenase activity was detectable in any zone of the growth plate, whereas control liver slices exhibited abundant enzyme activity. Thus the glycerol phosphate shuttle, one pathway whereby reducing equivalents are carried or shuttled from cytoplasmic nicotinamide adenine dinucleotide to the intramitochondrial respiratory chain, is entirely lacking in growth-plate chondrocytes. The lack of this enzyme, and the absence of the glycerol phosphate shuttle, may explain the high rate of lactate accumulation in the presence of ample oxygen concentration (aerobic accumulation) even though the Krebs cycle (tricarboxylic acid cycle) and electron transport are proceeding at normal rates. CLINICAL RELEVANCE: The growth plate is a complex structure whose metabolism is still not well understood. This study demonstrates that, in at least one metabolic pathway, the growth-plate chondrocyte does not resemble other normal cells. Only as we are able to formulate a more complete picture of the metabolism of the normal growth plate will be able to understand, and perhaps correct or prevent, those disease states in which growth-plate metabolism has bone away.