Mechanism of an active transport of calcium. Ethoxyformylation of sarcoplasmic reticulum vesicles

Diethyl pyrocarbonate inactivates CD39/ecto-ATPDase by modifying His-59

Diethyl pyrocarbonate (DEPC) in conditions that favour carbethoxylation of histidyl residues strongly inactivated E-type ATPase activity of a rat lung membrane preparation, as well as ecto-ATPase activity of rat vessels and human Epstein-Barr virus-transformed B lymphocytes. Inactivation of the enzyme (up to 70%) achieved at concentrations of DEPC below 0.5 mM could be fully reversed by 200 mM hydroxylamine at pH 7.5, thus confirming histidine-selective modification. UTP effectively protected the enzyme activity from DEPC inactivation. This was taken to indicate that the conformation adopted by the enzyme molecule upon substrate binding was not compatible with DEPC reaching and/or modifying the relevant histidyl residue. Substrate activation curves were interpreted to show the enzyme molecule to be inactive, at all substrate concentrations tested, when the target histidyl residue had been modified by DEPC. Comparison of known sequences of CD39-like ecto-ATP(D)ases with the results on inactivation by DEPC revealed His-59 and His-251 (according to the human CD39 sequence) as equally possible targets of the inactivating DEPC modification. Potato apyrase lacks a homologue for the former residue, while the latter is preserved in the enzyme sequence. Therefore, this enzyme was exposed to DEPC, and since hydrolysis of ATP and ADP by potato apyrase was insensitive to modification with DEPC, it was concluded that His-59 is the essential residue in CD39 that is affected by DEPC modification in a way that causes inactivation of the enzyme.

Modification of histidine 5 in sarcoplasmic reticulum Ca2+-ATPase by diethyl pyrocarbonate causes strong inhibition of formation of the phosphoenzyme intermediate from inorganic phosphate

Sarcoplasmic reticulum vesicles were modified with diethyl pyrocarbonate (DEPC), a histidine-modifying reagent. Phosphoenzyme formation from Pi in the Ca2+-ATPase (reversal of hydrolysis of the phosphoenzyme intermediate) was almost completely inhibited by this modification. Tight binding of F- and Mg2+ and high affinity binding of vanadate in the presence of Mg2+, both of which produce transition state analogs for phosphoenzyme formation from the magnesium-enzyme-phosphate complex, were also inhibited. Formation of the phosphoenzyme from acetyl phosphate in the forward reaction was only weakly inhibited, but hydrolysis of the phosphoenzyme was strongly inhibited. The enzyme was protected by tight binding of F- and Mg2+ or by high affinity binding of vanadate in the presence of Mg2+ against the DEPC-induced inhibition of phosphoenzyme formation from Pi. The enzyme was also protected by tight binding of F- and Mg2+ against the DEPC-induced inhibition of phosphoenzyme hydrolysis. Peptide mapping of the tryptic digests, detection of peptides containing DEPC-modified histidine by UV absorption at 240 nm, amino acid analysis, sequencing, and mass spectrometry showed that His-5 was a single major residue protected by the above transition state analogs against the modification with DEPC. These results indicate that modification of His-5 with DEPC is responsible for the DEPC-induced inhibition of phosphoenzyme formation from Pi and of phosphoenzyme hydrolysis and suggest that His-5 is located in or very close to the catalytic site in the transition state for phosphoenzyme formation from the magnesium-enzyme-phosphate complex and is likely involved in the catalytic process of this reaction step.

Evidence for an essential histidine residue in the Neurospora crassa plasma membrane H+-ATPase

The Neurospora crassa plasma membrane H+-ATPase is rapidly inactivated in the presence of diethyl pyrocarbonate (DEP). The reaction is pseudo-first-order showing time- and concentration-dependent inactivation with a second-order rate constant of 385-420 M-1.min-1 at pH 6.9 and 25 degrees C. The difference spectrum of the native and modified enzyme has a maximum near 240 nm, characteristic of N-carbethoxyhistidine. No change in the absorbance of the inhibited ATPase at 278 nm or in the number of modifiable sulfhydryl groups is observed, indicating that the inhibition is not due to tyrosine or cysteine modification, and the inhibition is irreversible, ruling out serine residues. Furthermore, pretreatment of the ATPase with pyridoxal phosphate/NaBH4 under the conditions of the DEP treatment does not inhibit the ATPase and does not alter the DEP inhibition kinetics, indicating that the inactivation by DEP is not due to amino group modification. The pH dependence of the inactivation reaction indicates that the essential residue has a pKa near 7.5, and the activity lost as a result of H+-ATPase modification by DEP is partially recovered after hydroxylamine treatment at 4 degrees C. Taken together, these results strongly indicate that the inactivation of the H+-ATPase by DEP involves histidine modification. Analyses of the inhibition kinetics and the stoichiometry of modification indicate that among eight histidines modified per enzyme molecule, only one is essential for H+-ATPase activity. Finally, ADP protects against inactivation by DEP, indicating that the essential residue modified may be located at or near the nucleotide binding site.