Involved in the synthesis of the GDP-mannose and dolichol-phosphate-mannose required for a number of critical mannosyl transfer reactions. D-mannose 6-phosphate = D-fructose 6-phosphate Binds 1 zinc ion per subunit. Inhibited by EDTA, Zn(2+), Cd(2+), DTT, p-chloromercuribenzoate and L-ascorbic acid (AsA). Optimum pH is 7.5. Optimum temperature is 48 degrees Celsius. Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; alpha-D-mannose 1-phosphate from D-fructose 6-phosphate: step 1/2. Not expressed in any organs under light (at protein level). Expressed at the last stage of senescence in old leaves. By sugar starvation, by dark and by 3-O-methyl-Glc (3-OMG). Down-regulated by sugars. Up-regulated by DCMU, an exogenous photosynthesis inhibitor. Belongs to the mannose-6-phosphate isomerase type 1 family.