Development and properties of fructose 1,6-bisphosphatase in the endosperm of castor-bean seedlings

Abstract

The activity of fructose 1,6-bisphosphatase (EC 3.1.3.11) in the fatty endosperm of castor bean (Ricinus communis) increases 25-fold during germination and then declines. The developmental pattern follows that of catalase, a marker enzyme for glucoeogenesis in this tissue. The enzyme at its peak of development was partially purified, and its properties were studied. It has an optimal activity at neutral pH (7.0-8.0). The apparent Km value for fructose 1,6-bisphosphate is 3.8 .times. 10-5 M. The activity is inhibited by AMP allosterically with an apparent Ki value of 2.2 .times. 10-4 M. The enzyme hydrolyzes fructose 1,6-bisphosphate and not ribulose 1,5-bisphosphate or sedoheptulose 1,7-bisphosphate. Treatment of the partially purified enzyme with acid leads to an 80% decrease in activity. The remaining activity is insensitive to AMP and has optimal activity at pH 6.7 and a high apparent Km value (2.5 .times. 10-4 M) for fructose 1,6-bisphosphate. Enzyme extracted from the tissue with water instead of buffer has a similar modification. The effect of acid explains the discrepancies between this report and previous ones on the properties of the enzyme in this tissue. The storage tissues of various fatty seedlings [Citrullus vulgaris, Zea mays, Cucumis sativus, Arachis hypogaea, Helianthus annuus] all contain a ''neutral'' fructose 1,6-bisphosphatase. The activities of the enzyme from some of the tissues are inhibited by AMP. The properties of the enzyme in fatty seedlings and in green leaves [Spinacia oleracea] are discussed in comparison with that in animal tissues.