Search

The Neurospora crassaplasma membrane H+-ATPase is inactivated by the periodate-oxidized nucleotides, oATP, oADP, and oAMP, with oAMP the most effective. Inhibition of the ATPase is essentially irreversible, because Sephadex G-50 column chromatography of the oAMP-treated ATPase does not result in a reversal of the inhibition. Inhibition of the ATPase by oAMP is protected against by the H+-ATPase substrate ATP, the product ADP, and the competitive inhibitors TNP (2′,3′-O-(2,4,6-trinitrocyclohexadienylidine)-ATP and TNP-ADP, suggesting that oAMP inhibition occurs at the nucleotide binding site of the enzyme. The rate of inactivation of the ATPase by oAMP is only slightly affected by EDTA, indicating that the oAMP interaction with the nucleotide binding site of the H+-ATPase occurs in the absence of a divalent cation. The protection against oAMP inhibition by ADP is likewise unaffected by EDTA. The inhibition of the ATPase by oAMP is absolutely dependent on the presence of acidic phospholipids or acidic lysophospholipids known to be required for H+-ATPase activity, suggesting that these lipids either aid in the formation of the nucleotide binding site or render it accessible. Incubation of the ATPase with Mg2+plus vanadate, which locks the enzyme in a conformation resembling the transition state of the enzyme dephosphorylation reaction, completely protects against inhibition by oAMP, suggesting that in this transition state conformation the nucleotide site either does not exist, or is inaccessible to oAMP. Labeling studies with [14C] oAMP indicate that the incorporation of 1 mol of oAMP is sufficient to cause complete inactivation of the ATPase.

The reaction of aldose reductase from human psoas muscle with either pyridoxal 5′-phosphate (PLP) or pyridoxal 5′-diphospho-5′-adenosine (PLP-AMP) results in a pseudo first-order 2-fold activation of the enzyme with the stoichiometric incorporation of 1 mol of either reagent per mol of enzyme. However, in addition to an increase in Vmaxthere was also an increase in Kmfor both substrate, DL-glyceraldehyde, and coenzyme, NADPH. This resulted in an overall decrease in catalytic efficiency (kcat/Km). Spectral analysis indicated that activation by both PLP and PLP-AMP was accompanied by Schiff's base formation and ε-pyridoxyllysine was identified in hydrolysates of the reduced enzyme PLP-complex. Digestion of either PLP-modified or PLP-AMP-modified aldose reductase with endoproteinase Lys-C followed by high performance liquid chromatography purification and amino acid sequencing of the pyridoxyllated peptide revealed that PLP and PLP-AMP had modified the same lysine residue. A 32-residue peptide containing the essential lysine was found to be highly homologous with a segment of the sequence of both human liver aldehyde reductase and rat lens aldose reductase. A tetrapeptide (Ile-Pro-Lys-Ser) containing the essential lysine was identical in all three enzymes. These results highlight the close structural similarity between members of the aldehyde reductase family.

Aldose reductase (ALR2) has been purified to homogeneity from human psoas muscle. From sodium dodecyl sulfate-polyacrylamide electrophoresis the enzyme is monomeric and has a molecular weight of 37,000. ALR2 catalyzes the primarily NADPH-dependent reduction of a wide variety of aldehydes, although the enzyme can also utilize NADH. The best substrates for ALR2 are aromatic aldehydes (e.g.pyridine-3-aldehyde; Km= 9 µM; kcat/Km= 150,000 s−1M−1), while among aldoses DL-glyceraldehyde is the preferred substrate (Km= 72 µM; kcat/Km= 17,250). Low (100 µM) concentrations of CaCl2and CaSO4cause a marked inhibition (90%) of ALR2 as do higher concentrations (0.2 M) of MgCl2. (NH4)2SO4caused a 2-fold activation of ALR2. The enzyme is also inhibited by quercetin and the commercially developed aldose reductase inhibitors alrestatin and sorbinil. ALR2 is inhibited only very slightly by sodium valproate and barbiturates. ALR2 cross-reacts immunologically with human brain and human placental aldose reductase and with ALR2 from monkey tissue. There is no precipitin cross-reaction of ALR2 with aldose reductases from other species nor with human aldehyde reductase 1 (ALR1) or with ALR1 from other species. The data show that human muscle is a new and relatively rich source of a monomeric NADPH/NADH reductase which is clearly identifiable as aldose reductase.