Biochemically, σ 1 is the specific non-observable initial substrate concentration, and η 1 is the reciprocal of specificity time for the non-observable reaction. The validity in the reduction of (7a)-(7b) to the differential-algebraic system (8)-( ) is dependent not only on the rate constants k -1 , k 1 and the catalytic constant k 2 , but also on the initial substrate and enzyme concentrations. It is generally assumed that s 1 ≈ s 0 1 during the initial buildup c 1 : this is known as the RSA . The timescale over which the buildup of c 1 occurs is given by t c1 , and was originally estimated to be