From my understanding, the specific activity (A, units: umol/min/mg protein) can be derived from the turnover number (k, units: 1/seg) and the molar mass of an enzyme (MW, units: g/mol):
A = k / MW / 60000
When I try to do this for several entries in the BRENDA database, I don't get an agreement, sometimes the equality is off by an order of magnitude. However I understand that there can be experimental errors involved, or I might be using the wrong MW (sometimes it is not clear what specific protein the value refers to).
Am I doing something wrong in this calculation?