Effects of Phospholipases on the Kinetic Properties of Rat Striatal Membrane‐Bound Tyrosine Hydroxylase

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Abstract
Rat striatal tyrosine hydroxylase can be isolated in both a soluble and a synaptic membrane-bound form. The membrane-bound enzyme, which exhibits lower Km for both tyrosine (7 .mu.M) and reduced pterin cofactor (110 .mu.M) relative to the soluble enzyme (47 .mu.M and 940 .mu.M, respectively), can be released from the membrane fraction with mild detergent, and concomitantly its kinetic properties revert to those of the soluble enzyme. Treatment of membrane-bound tyrosine hydroxylase with Clostridium perfringens phospholipase C increased the Km of the enzyme for tyrosine to 27 .mu.M and the Vmax by 60% without changing the Km for cofactor. In contrast, treatment of membrane-bound tyrosine hydroxylase with Vipera russelli phospholipase A2 increased the Km for tyrosine to 48 .mu.M, increased the Vmax and increased the Km for cofactor to 560 .mu.M. The enzyme remained bound to the membrane fraction following both phospholipase treatments. Addition of phospholipids to treated enzyme could partially reverse the effects of phospholipase A2 treatment, but not the effects of phospholipase C treatment. The kinetic properties of phospholipase-treated, detergent-solubilized tyrosine hydroxylase were identical to those of the control solubilized enzyme. Tyrosine hydroxylase appears to interact with synaptic membrane components to produce at least 2 separately determined consequences for the kinetic properties of the enzyme.

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