Fructose 2,6-Bisphosphate Hydrolyzing Enzymes in Higher Plants

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Abstract
The phosphatases that hydrolyze fructose 2,6-bisphosphate in a crude spinach (Spinacia oleracea L.) leaf extract were separated by chromatography on blue Sepharose, into three fractions, referred to a phosphatases I, II, and III, which were further purified by various means. Phosphatase I hydrolyzed fructose 2,6-bisphosphate, with a Km value of 30 micromolar, to a mixture of fructose 2-phosphate (90%) and fructose 6-phosphate (10%). It acted on a wide range of substrates and had a maximal activity at acidic pH. Phosphatase II specifically recognized the osyl-link of phosphoric derivatives and had more affinity for the .beta.-anomeric form. Its apparent Km for fructose 2,6-bisphosphate was 30 micromolar. It most likely corresponded to the fructose-2,6-bisphosphatase described by F. D. Macdonald, Q. Chou, and B. B. Buchanan ([1987] Plant Physiol 85: 13-16). Phosphatase III co-purified with phosphofructokinase 2 and corresponded to the specific, low-Km (24 nanomolar) fructose-2,6-bisphosphatase purified and characterized by Y. Larondelle, E. Mertens, E. Van Schaftingen, and H. G. Hers ([1986] Eur J Biochem 161: 351-357). Three similar types of phosphatases were present in a crude extract of Jerusalem artichoke (Helianthus tuberosus) tuber. The concentration of fructose 2,6-bisphsophate decreased at a maximal rate of 30 picomoles per minute and per gram of fresh tissue in slices of Jerusalem artichoke tuber, upon incubation in 50 millimolar mannose. This rate could be accounted for by the maximal extractable activity of the low-Km fructose-2,6-bisphosphatase. A new enzymic method for the synthesis of .beta.-glucose 1,6-bisphosphate from .beta.-glucose 1-phosphate and ATP is described.