The enzymology of adenosine triphosphate sulphurylase from spinach leaf tissue. Kinetic studies and a proposed reaction mechanism

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Abstract
1. Sulphate-dependent PPi–ATP exchange, catalysed by purified spinach leaf ATP sulphurylase, was correlated with the concentration of MgATP2− and MgP2O72−; ATP sulphurylase activity was not correlated with the concentration of free Mg2+. 2. Sulphate-dependent PPi–ATP exchange was independent of PPi concentration, but dependent on the concentration of ATP and sulphate. The rate of sulphate-dependent PPi–ATP exchange was quantitatively defined by the rate equation applicable to the initial rate of a bireactant sequential mechanism under steady-state conditions. 3. Chlorate, nitrate and ADP inhibited the exchange reaction. The inhibition by chlorate and nitrate was uncompetitive with respect to ATP and competitive with respect to sulphate. The inhibition by ADP was competitive with respect to ATP and non-competitive with respect to sulphate. 4. ATP sulphurylase catalysed the synthesis of [32P]ATP from [32P]PPi and adenosine 5′-sulphatophosphate in the absence of sulphate; some properties of the reaction are described. Enzyme activity was dependent on the concentration of PPi and adenosine 5′-sulphatophosphate. 5. The synthesis of ATP from PPi and adenosine 5′-sulphatophosphate was inhibited by sulphate and ATP. The inhibition by sulphate was non-competitive with respect to PPi and adenosine 5′-sulphatophosphate; the inhibition by ATP was competitive with respect to adenosine 5′-sulphatophosphate and non-competitive with respect to PPi. It was concluded that the reaction catalysed by spinach leaf ATP sulphurylase was ordered; expressing the order in the forward direction, MgATP2− was the first product to react with the enzyme and MgP2O72− was the first product released. 6. The expected exchange reaction between sulphate and adenosine 5′-sulphatophosphate could not be demonstrated.