Pyridoxylation of essential lysine residues of mitochondrial adenosine triphosphatase

Affinity labeling of the guanine nucleotide binding site of transducin by pyridoxal 5'-phosphate

Transducin (T), a guanine nucleotide binding regulatory protein composed of alpha-, beta-, and gamma-subunits, serves as an intermediary between rhodopsin and cGMP phosphodiesterase during signaling in the visual process. Pyridoxal 5'-phosphate (PLP), a reagent that has been used to modify enzymes that bind phosphorylated substrates, was probed here as an affinity label for T. PLP inhibited the guanine nucleotide binding activity of T in a concentration dependent manner, and was covalently incorporated into the protein in the presence of [3H]NaBH4. Approximately 1 mol of 3H was bound per mol of T. GTP and GTP analogs appreciably hindered the incorporation of 3H to T, suggesting that PLP specifically modified the protein active site. Interestingly, PLP modified both the alpha- and beta-subunits of T. Moreover, PLP in the presence of GDP behaved as a GTP analog, since this mixture was capable of dissociating T from T:photoactivated rhodopsin complexes.

Enhancement of aldose reductase activity by modification of an active site lysine: a possible mechanism for in vivo activation

Reaction of aldose reductase (ALR2) from pig muscle with pyridoxal 5'-phosphate (pyridoxal-P) and other lysine modifying reagents resulted in activation of the enzyme. The activation by pyridoxal-P showed pH and concentration dependence that was prevented by incubation with NADPH and various cofactor analogues but not by the aldehyde substrate. Spectral analysis of the reaction showed characteristic peaks associated with Schiff's base formation between a lysine amino group and the aldehyde of pyridoxal-P. Subsequent reduction produced spectra characteristic of a phosphopyridoxyllysine bond. Phosphopyridoxyllysine was isolated by amino acid analysis of modified ALR2. Determination of the stoichiometry of bound phosphopyridoxyllysine indicated one mole of pyridoxal-P per mole of enzyme under conditions that produced maximal activation. A single [3H]phosphopyridoxyllysine containing peptide was isolated by high performance liquid chromatography after enzymatic cleavage of the modified enzyme. This 34 residue peptide exhibited considerable sequence homology to the region comprising residues 242 to 275 of human liver ALR1 and a similar region in rat lens ALR2, human muscle ALR2 and human placental ALR2. The activation of ALR2 via formation of a Schiff's base suggests a possible mechanism of activation of the enzyme in vivo by glucose.

Inactivation of the mitochondrial ATPase inhibitor protein by chemical modification with diethylpyrocarbonate

Modification of histidine residue(s) by diethylpyrocarbonate treatment of submitochondrial particles obtained by sonication results in inhibition of ATPase activity and stimulation of oligomycin-sensitive H+ conduction. The inhibition of the ATPase (EC 3.6.1.3) activity persisted in F1 isolated from diethylpyrocarbonate-treated submitochondrial particles, which exhibited the absorbance spectrum of modified histidine. Thus the inhibition of the ATPase activity results from histidine modification in F1 subunits. Removal of the natural inhibitor protein from submitochondrial particles resulted in stimulation of proton conduction. After removal of F1 inhibitor protein from the particles the stimulatory effect exerted by diethylpyrocarbonate treatment on proton conduction was lost. Reconstitution experiments showed that purified F1 inhibitor protein lost, after histidine modification, its capacity to inhibit the ATPase activity and proton conduction. These observations show that the stimulation of proton conduction by the ATPase complex effected by diethylpyrocarbonate treatment results from histidine modification in F1 inhibitor protein.

[Use of chemical probes in the study of F1-ATPases]

The purpose of the present review is to discuss in brief the use of chemical probes for the study of the structure and the function of F1-ATPases. Special focus is brought on probes that bind covalently to the proteins.

The inhibition of mitochondrial F1-ATPase by 1,5-difluoro-2,4-dinitrobenzene

1,5-Difluoro-2,4-dinitrobenzene completely inhibits F1-ATPase when it is used at micromolar concentrations and the inhibitor/enzyme molar ratio is equal to 3. The inhibition can be reversed by dithiothreitol treatment. 7-Chloro-4-nitrobenzofurazan treatment of F1-ATPase does not prevent the reaction of the enzyme with 1,5-difluoro-2,4-dinitrobenzene. The 1,5-difluoro-2,4-dinitrobenzene-induced inhibition is thought to be a result of the modification of a tyrosine residue with pK 9.1.

Changes in chemical properties of mitochondrial adenosinetriphosphatase upon removal of tightly bound nucleotides

The removal of tightly bound nucleotides from mitochondrial F1-ATPase was found to affect the inhibition by ADP and chemical reactivity toward 7-chloro-4-nitro-2,1,3-benzoxadiazole (NBD-C1) and sulfhydryl reagents. Preincubation of nucleotide-depleted F1 with 40 microM ADP in the presence of ethylenediaminetetraacetic acid (EDTA) resulted in a 51% inhibition of the steady-state level of ATPase activity whereas only a 25% inhibition was observed for native F1. Both partially inhibited states of the enzyme could be reversed by the subsequent addition of ATP. Measurement of [14C]ADP binding to nucleotide-depleted F1 in the presence of EDTA reveals three equivalent ADP binding sites with a Kd of 0.45 microM, and a fourth site of lower affinity. The sulfhydryl reagents 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and N-ethylmaleimide (NEM) were found to inhibit the ATPase activity of nucleotide-depleted F1 but not native F1 or nucleotide-depleted F1 in the presence of ADP or ATP. Polyacrylamide gel electrophoresis of nucleotide-depleted F1 labeled with [14C]NEM gave a 2-fold increase in incorporation into the (alpha + beta) subunits and a 7-fold increase in label in the gamma subunit after 90 min compared to when ADP was present during the reaction. ADP binding to the noncatalytic sites enhanced the rate of inhibition of nucleotide-depleted F1 by NBD-C1 about 2-fold while retarding the subsequent intramolecular transfer from an essential phenol group to an amino group about 2.8-fold. The results suggest a conformational change in F1 caused by changes in nucleotide--protein interaction at the noncatalytic sites.