Soybean Leaf Urease: A Seed Enzyme?

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Abstract
The soybean (G. max L. [Merrill]) cv. Itachi has 0.2-0.3% the urease activity found in developing embryos of a normal line, Prize. The hydroxyurea sensitivity and pH preference of this basal seed urease indicate that it represents a unique enzyme rather than an unusually low level of the normal seed urease. ''Itachis'' seed urease is less sensitive to hydroxyurea inhibition (65-80% inhibition) than Prize seed urease (85-95% inhibition) and is more active at pH 6.1 and 8.8 than at 7.4, whereas the normal seed urease is least active at pH 8.8. Both properties of the basal seed urease are in agreement with the bahavior of the leaf urease in extracts of Prize and ''Itachi'' leaves. Neither the leaf urease nor the ''Itachi'' seed urease is immunoprecipitated by affinity-purified seed urease antibodies. However, when antibody is in excess, Staphylococcus aureus (Cowan) cell walls containing protein A can precipitate soluble antibody-urease complexes (47-68% of total enzyme) from both leaf (''Itachi'' and Prize) and ''Itachi'' seed extracts. Under identical conditions, > 90% of Prize seed urease is precipitated. At a 100-fold dilution of antibody, 60% of Prize seed urease is still antibody-complexed while the antibody recognition of the leaf or ''Itachi'' seed urease is reduced to 2-24%. The cell culture urease also resembles leaf urease by the criteria of pH preference, hydroxyurea sensitivity and recognition by seed urease antibodies. In the presence of cycloheximde, Ni stimulates cell culture urease levels (14- or 35-fold depending on assay pH) indicating that cell cultures make a preponderance of apourease under Ni-limiting conditions. The ureases of leaf, cell culture and ''Itachi'' seeds are more closely related to each other than they are to the abundant (Prize) seed urease, suggesting that the 3 tissues either contain an identical urease or related tissue-specific isozymes. This 2nd form of urease may have an assimilatory role since it is found in both leaf and seed sink tissues and is required for urea assimilation in cell culture.