A carbamoyl-transfer reaction has been demonstrated in the urease-catalyzed hydrolysis of urea, using a thymol blue buffer to give a sensitive spectrophotometric measure of acidity changes. This demonstration offers further support for the proposition that carbamate is the first product of the urease-catalyzed hydrolysis of urea. The inhibition of urease by aceto-Jack bean urease (urea amidohydrolase, EC 3.5.1.5) has a cherished place in the enzymologist's heart, since it was responsible for the death blow to the proposition that the protein was merely a carrier of the catalytic species (Northrop, 1961). Nonetheless, the enzyme remains ill understood and is deserving of a detailed and intensive mechanistic attack. Sumner (1951) described urease as "absolutely specific" and until recently only two additional substrates, hydroxyurea and dihydroxyurea, have been

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