Adenine nucleotides and magnesium ions in relation to control of mammalian cerebral-cortex hexokinase.
1. The kinetics of inhibition of brain soluble cytoplasmic hexokinase by ADP were examined in relation to variations in the concentrations of Mg(2+) and ATP. The type of inhibition observed was dependent on the Mg(2+)/ATP ratio. 2. ADP at Mg(2+)/ATP ratios 2:1 exhibited inhibition of the ;mixed' type; at Mg(2+)/ATP ratios 1:1 the inhibition appeared to be competitive with regard to ATP. 3. Inhibition by free ATP was observed when the Mg(2+)/ATP ratio was less than 1:1. The inhibition was also of the ;mixed' type with respect to MgATP(2-). 4. The inhibitions due to ADP and to free ATP were not additive. The results suggested that there may be up to four sites in the soluble enzyme: for glucose, glucose 6-phosphate, ADP and MgATP(2-). 5. The ;free' non-particulate intracellular Mg(2+) concentration was measured and concluded to be about 1.5mm. 6. The concentrations in vivo of Mg(2+) and ATP likely to be accessible to a cytoplasmic enzyme are suggested to be below those that yield maximum hexokinase rates in vitro. The enzymic rates were measured at relevant suboptimum concentrations of Mg(2+) and ATP in the presence of ADP. Calculations that included non-competitive inhibition due to glucose 6-phosphate (56-65% at 0.25mm) resulted in net rates very similar to the measured rates for overall glycolysis. This system may therefore provide a basis for effective control of cerebral hexokinase.
D000227:Adenine Nucleotides; D000255:Adenosine Triphosphate; D000818:Animals; D001665:Binding Sites; D001923:Brain Chemistry; D002417:Cattle; D002540:Cerebral Cortex / Q000166:cytology / Q000201:enzymology*; D055598:Chemical Phenomena; D011232:Chemical Precipitation; D002621:Chemistry; D003593:Cytoplasm / Q000201:enzymology*; D006168:Guinea Pigs; D006593:Hexokinase / Q000037:antagonists & inhibitors* / Q000302:isolation & purification; D006600:Hexosephosphates; D007700:Kinetics; D008274:Magnesium / Q000032:analysis; D008722:Methods; D000644:Quaternary Ammonium Compounds; D012995:Solubility; D013053:Spectrophotometry; D014461:Ultracentrifugation
D000227:Adenine Nucleotides; D006600:Hexosephosphates; D000644:Quaternary Ammonium Compounds; D000255:Adenosine Triphosphate; D006593:Hexokinase; D008274:Magnesium
J Biol Chem. 1964 Jan;239:31-42
14114860
Anal Chim Acta. 1966 Jan;34(1):17-23
5901635
Biochim Biophys Acta. 1961 Dec 9;54:330-8
14478319
Biochem J. 1953 Aug;55(1):170-1
13093635
Biochem J. 1957 Oct;67(2):245-50
13471540
J Biol Chem. 1954 Feb;206(2):925-36
13143054
Biochem J. 1968 Jan;106(1):193-201
5721456
Biochem J. 1959 Feb;71(2):388-95
13628582
J Biol Chem. 1964 Jan;239:18-30
14114842
J Biol Chem. 1954 Oct;210(2):597-606
13211596
Methods Biochem Anal. 1966;14:1-52
5328503
Biochem J. 1967 Jul;104(1):286-92
6035519
J Neurochem. 1959 Dec;5:1-17
13796106
Biochemistry. 1964 Jan;3:18-26
14114498
J Biol Chem. 1965 Dec;240(12):4682-8
4221248
Anal Biochem. 1967 Nov;21(2):169-77
6082096
Biochem J. 1963 May;87:248-55
13939322
Nature. 1961 Jul 22;191:381-2
13785572
J Biol Chem. 1962 May;237:1661-7
13895500
J Biol Chem. 1962 Jun;237:1758-63
13867298
Biochem J. 1951 Aug;49(3):339-47
14858341
Biochem J. 1962 Aug;84:225-32
13863711
J Biol Chem. 1953 Jul;203(1):273-92
13069512