Differences in catalytic properties between cerebral cytoplasmic and mitochondrial hexokinases

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Abstract
Clear kinetic differences between cytoplasmic and mitochondrial forms of type-I ox cerebral hexokinase were demonstrated from experiments performed under identical conditions on 3 (cytoplasmic, bound mitochondrial and solubilized mitochondrial) preparations of the enzyme. The Km for glucose .**GRAPHIC**. was consistent, that for MgATP2- .**GRAPHIC**. was lower in the cytoplasmic than in the 2 mitochondrial preparations. The substrate dissociation constants .**GRAPHIC**. were both higher in the cytoplasmic than in the mitochondrial preparations. A further difference in the substrate kinetic patterns was that .**GRAPHIC**. for the cytoplasmic enzyme, in contrast with the mitochondrial enzyme, where .**GRAPHIC**. was clearly not equal to .**GRAPHIC**. Dead-end inhibition produced by N-acetylglucosamine and by AMP also exhibited different quantitative kinetic patterns for the 2 enzyme sources. Both inhibitions gave Ki values similar or equal to those of Ki'' for the cytoplasmic activity; Ki was clearly not equal to Ki'' for the mitochondrial activity. All of these studies demonstrated the similarity of the 2 mitochondrial activities (particulate and solubilized), which were both clearly different from the cytoplasmic activity. The analysis gives a practical example of previous theoretical treatment on the derivation of true inhibition constants. The results are discussed in terms of the function of cerebral hexokinases.