Abstract
SUMMARY Urea Inhibition of fifetallo-enzymes--From the data here presented, it appears that, in general, urea is relatively ineffective as an inhibitor of those metallo-enzymes whose metal component is involved in the formation of the enzyme-substrate complex. Thus, hexokinase and alkaline phosphatase which show an ab- solute dependence upon the presence of Mg++, and the ferri- porphyrin proteins, peroxidase and cytochrome oxidase, are com- pletely unaffected by urea even at 2 M concentration. Uricase, a copper-protein (45), carboxypeptidase, a zinc protein (46), catalase, a hemoprotein, and inorganic pyrophosphatase which is dependent upon Mg++ in the medium are inhibited only at very high concentrations of urea. Much evidence suggests that the metal of many metal-requiring or metal-containing enzymes serves as a bridge between enzyme and substrate in the formation of the enzyme-substrate complex (47). It is extremely unlikely that urea can prevent chelation or coordination of the enzyme or substrate by a metal. Similarly, it is unlikely that urea could competitively inhibit the action of enzymes on inorganic sub- strates such as pyrophosphate, sulfite or peroxide since, in these instances, electrostatic forces rather than hydrogen-bonding or van der Waals’ forces must be involved in formation of the en- zyme-substrate complex. Xanthine oxidase has been found to be competitively inhibited by low levels of urea. Analysis of urea inhibition of a series of diverse enzymes indicates that urea is, in general, a competitive inhibitor of enzymes which act upon organic substrates. Guani- dine is much more effective than urea as a competitive inhibitor of the same group of enzymes. Relatively higher concentrations of urea are required to effect inhibition of enzymes which act upon inorganic substrates and such inhibitions are either “non- competitive” or “uncompetitive.” In general, enzymes which require metal ions as cofactors or which contain a metal as an integral part of the enzyme molecule were found to be resistant to inhibition by urea.
Highlights
The inhibition of enzyme activity would seem to reflect the attachment of urea at the active site, in view of the relatively low concentrations of urea required for inhibition, and of the competitive nature of the inhibition
Xanthine oxidase has been found to be competitively inhibited by low levels of urea
Analysis of urea inhibition of a series of diverse enzymes indicates that urea is, in general, a competitive inhibitor of enzymes which act upon organic substrates
Summary
Lactic dehydrogenase, histidase, horseradish peroxidase, alkaline phosphatase, acid phosphatase, tyr-Service and by Contract AT-(40-l)-289 between Duke University and the United States Atomic Energy Commission.t Senior Research Fellow, NationalInstitutes of Health, United States Public Health Service.osinasc, liver alcohol dehydrogenase, yeast alcohol dehydrogenase, uricase, and carboxypeptidase were obtained fromWorthington Biochemical Corporation. Lactic dehydrogenase, histidase, horseradish peroxidase, alkaline phosphatase, acid phosphatase, tyr-. Service and by Contract AT-(40-l)-289 between Duke University and the United States Atomic Energy Commission. Institutes of Health, United States Public Health Service. Osinasc, liver alcohol dehydrogenase, yeast alcohol dehydrogenase, uricase, and carboxypeptidase were obtained from. The other enzymes were prepared as described below. Xanthine Ox&se-This enzyme was purified as previously described [13]. Assays were conducted in 0.05 M potassium phosphate, pH 7.8, containing 0.005% Versene Fe-3l, at 25” unless otherwise specified. Enzyme activity was usually assayed according to Kalckar [14] with xanthine as substrate by observing the increase in absorbancy at 295 mp. When salicylaldehyde was the substrate, salicylic acid production was estimated from the increase in absorbancy at 290 rnp
Sign-in/Register to view highlights & summary 
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
