Purification, properties and substrate specificity of adenosine triphosphate sulphurylase from spinach leaf tissue

Abstract

1. ATP sulphurylase was purified up to 1000-fold from spinach leaf tissue. Activity was measured by sulphate-dependent [32P]PPi–ATP exchange. The enzyme was separated from Mg2+-requiring alkaline pyrophosphatase (which interferes with the PPi–ATP-exchange assay) and from other PPi–ATP-exchange activities. No ADP sulphurylase activity was detected. 2. Sulphate was the only form of inorganic sulphur that catalysed PPi–ATP exchange; Km (sulphate) was 3.1mm, Km (ATP) was 0.35mm and the pH optimum was 7.5–9.0. The enzyme was insensitive to thiol-group reagents and required either Mg2+ or Co2+ for activity. 3. The enzyme catalysed [32P]PPi–dATP exchange; Km (dATP) was 0.84mm and V (dATP) was 30% of V (ATP). Competition between ATP and dATP was demonstrated. 4. Selenate catalysed [32P]PPi–ATP exchange and competed with sulphate; Km (selenate) was 1.0mm and V (selenate) was 30% of V (sulphate). No AMP was formed with selenate as substrate. Molybdate did not catalyse PPi–ATP exchange, but AMP was formed. 5. Synthesis of adenosine 5′-[35S]sulphatophosphate was demonstrated by coupling purified ATP sulphurylase and Mg2+-dependent alkaline pyrophosphatase (also prepared from spinach) with [35S]sulphate and ATP as substrates; adenosine 5′-sulphatophosphate was not synthesized in the absence of pyrophosphatase. Some parameters of the coupled system are reported.