Effect of spermine on tyrosine hydroxylase activity before and after phosphorylation by cyclic AMP-dependent protein kinase.
The effect of spermine on tyrosine hydroxylase ( TH) activity purified from bovine adrenal medulla was examined before and after phosphorylation by the catalytic subunit of cyclic AMP-dependent protein kinase (A-kinase). Before phosphorylation, spermine (less than 1 mM) inhibited the enzymatic activity, and negative cooperative effect of spermine on TH (Hill coefficient = 0.7) was observed from the kinetic analysis concerning 6-methyl-5,6,7,8-tetrahydropterin (6MPH4). Spermine interacted noncompetitively toward tyrosine and the Ki for spermine was calculated to be 68 microM. Phosphorylation abolished the ability of spermine to inhibit TH activity in a negative cooperative manner against the pterin cofactor, and also increased four-fold the Ki value against the substrate. These results suggest that spermine may inhibit TH activity by interacting with the pterin binding site of the enzyme molecule in a manner of negative cooperativity, and that this inhibition is reversed by the conformational change of regulatory domain of TH after phosphorylation by A-kinase.[1]References
- Effect of spermine on tyrosine hydroxylase activity before and after phosphorylation by cyclic AMP-dependent protein kinase. Kiuchi, K., Kiuchi, K., Togari, A., Nagatsu, T. Biochem. Biophys. Res. Commun. (1987) [Pubmed]
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