Fig. (4). Titration of Syk enzyme at 0.1µM substrate Various concentrations of Syk enzyme were run using different substrates at a concentration of 0.1µM or 0.01 µM. The TK substrate-biotin showed the greatest performance under either concentration, with the optimal condition (1.36nM Syk, 0.1µM TK substrate-biotin) producing a signal to background ratio of 21.5.