As a step toward analyzing the serine biosynthetic pathway in mammals, we have studied the properties of phosphoserine aminotransferase, the second step-catalyzing enzyme. The K(m) values for 3-phosphohydroxypyruvate and L-phosphoserine are 5 and 35 microM, respectively, and those for glutamate and alpha-ketoglutarate are 1.2 and 0.8 mM, respectively. The product inhibition studies strengthened the support for a ping-pong mechanism and allowed evaluation of Ki values for the four substrates. The equilibrium constant evaluated from the kinetic parameters is approximately 40. Additionally, some physical properties relative to the bound coenzyme and the secondary structure were investigated. The results are consistent with a structural relationship between the Escherichia coli enzyme and the mammalian enzyme. The mammalian enzyme has specific kinetic parameters, the determination of which is a prerequisite to analyzing the serine biosynthetic pathway in mammals.