Albumins activate peptide hydrolysis by the bifunctional enzyme LTA4 hydrolase/aminopeptidase.
Albumins from several species activated the bifunctional, Zn2+ metalloenzyme amino-peptidase/leukotriene A4 hydrolase (EC 3.3.2.6). Bovine serum albumin, 1 mg/mL, increased hydrolysis of L-proline-p-nitroanilide and leucine-enkephalin by 12-fold and 7-fold, respectively. The apparent Km for L-proline-p-nitroanilide was inversely proportional to the albumin concentration from 0 to 1 mg/mL, declining from 9.4 to 0.7 mM without an appreciable change in apparent Vmax. These data imply a random activation process in which the enzyme-activator complex is catalytically dominant. Hill plots indicated a 1:1 stoichiometric relationship between albumin and enzyme. Secondary plots of slope versus the reciprocal of albumin concentration indicated that it binds to the enzyme with an affinity constant of 0.9 microM. The pH optimum of the nonactivated enzyme occurred at pH 8; the albumin-activated enzyme had an optimum near pH 7. Neither ultrafiltration nor dialysis of albumin altered its activating effect, but boiling abolished it. Albumin did not affect other cytosolic or microsomal leucine aminopeptidases, or gamma-glutamyltransferase. Albumin functions as a nonessential activator, since enzymatic activity was always detectable in its absence. Chloride ions, which activate other Zn2+ metalloenzymes, also activated leukotriene A4 hydrolase/aminopeptidase with an EC50 = 50 mM, increasing its initial velocity 2.2-fold in the absence of albumin. Zn2+ activated the enzyme, increasing its apparent Vmax but not its apparent Km, suggesting it replaced Zn2+ lost from the active site, especially at acidic pH. At concentrations greater than 30-50 microM, Zn2+ was inhibitory. Albumin mitigated the effect of chloride, but not the effect of Zn2+ or that of the competitive inhibitor, captopril.(ABSTRACT TRUNCATED AT 250 WORDS)
Orning
L
L
Department of Pharmacology, University of Colorado Health Sciences Center, Denver 80262.
Fitzpatrick
F A
FA
D000418:Albumins / Q000378:metabolism*; D000626:Aminopeptidases / Q000378:metabolism*; D002216:Captopril / Q000494:pharmacology; D004789:Enzyme Activation; D004851:Epoxide Hydrolases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007975:Leukotriene B4 / Q000378:metabolism; D011994:Recombinant Proteins; D012965:Sodium Chloride / Q000494:pharmacology; D013379:Substrate Specificity; D013431:Sulfates / Q000494:pharmacology; D015032:Zinc / Q000494:pharmacology; D019287:Zinc Sulfate
D000418:Albumins; D011994:Recombinant Proteins; D013431:Sulfates; D007975:Leukotriene B4; D012965:Sodium Chloride; D019287:Zinc Sulfate; D002216:Captopril; D004851:Epoxide Hydrolases; D000626:Aminopeptidases; D015032:Zinc; C043134:leukotriene A4 hydrolase