The structural requirements for binding to the glucose/sorbose-transport system in the human erythrocyte were explored by measuring the inhibition constants, K(i), for specifically substituted analogues of d-glucose when l-sorbose was the penetrating sugar. Derivatives in which a hydroxyl group in the d-gluco configuration was inverted, or replaced by a hydrogen atom, at C-1, C-2, C-3, C-4 or C-6 of the d-glucose molecule, all bound to the carrier, confirming that no single hydroxyl group is essential for binding to the carrier. The binding and transport of 1-deoxy-d-glucose confirmed that the sugars bind in the pyranose form. The relative inhibition constants of d-glucose and its deoxy, epimeric and fluorinated analogues are consistent with the combination of beta-d-glucopyranose with the carrier by hydrogen bonds at C-1, C-3, probably C-4, and possibly C-6 of the sugar. Both polar and non-polar substituents at C-6 enhance the affinity of d-glucose derivatives relative to d-xylose, and d-galactose derivatives relative to l-arabinose, and it is suggested that the carrier region around C-6 of the sugar may contain both hydrophobic and polar binding groups. The spatial requirements at C-1, C-2, C-3, C-4 and C-6 were explored by comparing the relative binding of d-glucose and its halogeno and O-alkyl substituents. The carrier protein closely approaches the sugar except at C-3 in the d-gluco configuration, C-4 and C-6. d-Glucal was a good inhibitor, showing that a strict chair form is not essential for binding. 3-O-(2',3'-Epoxypropyl)-d-glucose, a potential substrate-directed alkylating agent, bound to the carrier, but did not inactivate it.

6-O-methyl-, 6-O-propyl-, 6-O-pentyl- and 6-O-benzyl-D-galactose, and 6-O-methyl-, 6-O-propyl- and 6-O-pentyl-D-glucose inhibit the glucose-transport system of the human erythrocyte when added to the external medium. Penetration of 6-O-methyl-D-galactose is inhibited by D-glucose, suggesting that it is transported by the glucose-transport system, but the longer-chain 6-O-alkyl-D-galactoses penetrate by a slower D-glucose-insensitive route at rates proportional to their olive oil/water partition coefficients. 6-O-n-Propyl-D-glucose and 6-O-n-propyl-D-galactose do not significantly inhibit L-sorbose entry or D-glucose exit when present only on the inside of the cells whereas propyl-beta-D-glucopyranoside, which also penetrates the membrane slowly by a glucose-insensitive route, only inhibits L-sorbose entry or D-glucose exit when present inside the cells, and not when on the outside. The 6-O-alkyl-D-galactoses, like the other nontransported C-4 and C-6 derivatives, maltose and 4,6-O-ethylidene-D-glucose, protect against fluorodinitrobenzene inactivation, whereas propyl beta-D-glucopyranoside stimulates the inactivation. Of the transported sugars tested, those modified at C-1, C-2 and C-3 enhance fluorodinitrobenzene inactivation, where those modified at C-4 and C-6 do not, but are inert or protect against inactivation. An asymmetric mechanism is proposed with two conformational states in which the sugar binds to the transport system so that C-4 and C-6 are in contact with the solvent on the outside and C-1 is in contact with the solvent on the inside of the cell. It is suggested that fluorodinitrobenzene reacts with the form of the transport system that binds sugars at the inner side of the membrane. An Appendix describes the theoretical basis of the experimental methods used for the determination of kinetic constants for non-permeating inhibitors.

A rapid and convenient chemical assay for the enzyme d-glucose 6-phosphate-1l-myoinositol 1-phosphate cyclase is described. The 1l-myoinositol 1-phosphate formed enzymically was oxidized with periodic acid liberating inorganic phosphate, which was assayed. myoInositol 2-phosphate can be assayed in the same way. Glucose 6-phosphate and other primary phosphate esters gave only very small quantities of inorganic phosphate under the conditions described. The K(m) of the enzyme for d-glucose 6-phosphate, 7.5+/-2.5x10(-4)m, was identical with that measured by the radiochemical method. 2-Deoxy-d-glucose 6-phosphate was a powerful competitive inhibitor, K(i) 2.0+/-0.5x10(-5)m, but was not a substrate for the enzyme.

6-O-Alkyl-d-galactoses competitively inhibit the erythrocyte sugar-transport system when added to the outside of the cells, but not to the inside. n-Propyl beta-d-glucopyranoside competitively inhibits the system on the inside of the cells, but not on the outside. A model for sugar transport is proposed.

After removal of tightly bound NAD(+) by using charcoal, a preparation of d-glucose 6-phosphate-1 l-myoinositol 1-phosphate cyclase catalysed the reduction of 5-keto-d-glucitol 6-phosphate and 5-keto-d-glucose 6-phosphate by [4-(3)H]NADH to give [5-(3)H]-glucitol 6-phosphate and [5-(3)H]glucose 6-phosphate respectively. The position of the tritium atom in the latter was shown by degradation. Both enzyme-catalysed reductions were strongly inhibited by 2-deoxy-d-glucose 6-phosphate, a powerful competitive inhibitor of inositol cyclase. The charcoal-treated enzyme preparation also converted 5-keto-d-glucose 6-phosphate into [(3)H]myoinositol 1-phosphate in the presence of [4-(3)H]NADH, but less effectively. These partial reactions of inositol cyclase are interpreted as providing strong evidence for the formation of 5-keto-d-glucose 6-phosphate as an enzyme-bound intermediate in the conversion of d-glucose 6-phosphate into 1 l-myoinositol 1-phosphate. The enzyme was partially inactivated by NaBH(4) in the presence of NAD(+). Glucose 6-phosphate did not increase the inactivation, and there was no inactivation in the absence of NAD(+). There was no evidence for Schiff base formation during the cyclization. d-Glucitol 6-phosphate (l-sorbitol 1-phosphate) was a good inhibitor of the overall reaction. It did not inactivate the enzyme. The apparent molecular weight of inositol cyclase as determined by Sephadex chromatography was 2.15x10(5).