How can a reaction exhibit first order kinetics when there is more than one reactant (the other being water)

This is for the hydrolysis of p-nitrophenyl phosphate by alkaline phosphatase, whereby the enzyme is in a solution of distilled water. We also add glycine buffer to the solution. It exhibits first-order kinetics, even though there is a second reactant (being water) - how is this possible?




It is because water is the solvent in the reaction and therefore its concentration remains practically constant throughout the course of the reaction. Hence, it is absorbed in the rate constant: 


rate=k[p-nitrophenyl phosphate]$^1$[H$\_2$O]$^x$=k'[p-nitrophenyl phosphate]$^1$


Hence, we say the reaction is peudo-first order. Also, in general, you could have a reaction with two reactants and it could still be first order overal since it could be 0 order in reactant A and first order in reactant B.


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