48 

 mM) and a lower maximum velocity (V^g^^ = 0.008 mmol/1 iter/min) with DL-DOPA 

 than with catechol (K^ = 3.09 mM and V^^^ = 0.041 mmol/1 iter/min) (Table 

 2). Similar results were also noted for the Florida lobster PPO (Table 

 2). These two PPOs had a higher affinity for catechol than for DL-DOPA. 

 Using DL-DOPA as a substrate, Simpson et al . (1987, 1988a) showed that 

 pink shrimp PPO had a lower Michael is constant (1.6 mM) than white shrimp 

 (2.8 mM). Rolle et al . (1991) characterized grass prawn PPO and they 

 determined the K^ of enzyme was 4.45 mM when DL-DOPA was used as substrate. 

 Summers (1967) showed that blood PPO from fiddler crab had a K^ value of 

 0.50 mM. 



Australian lobster PPO exhibited a higher affinity for DL-DOPA and 

 catechol than Florida lobster PPO. However, the latter showed a greater 

 rate for oxidizing DL-DOPA and catechol than the former. It has been 

 reported by Lavollay et al . (1963) that no relationship could be found 

 between K^ and V^^^ values obtained for a substrate with a given PPO 

 preparation. Instead, the term of V^^yK^ has been recommended by some 

 authors (Lavollay et al . , 1963; Pollock, 1965) to express the efficiency 

 of a given substrate for a given enzyme. Table 2 indicates that Florida 

 lobster PPO not only showed a higher specific activity but also had a 

 higher turnover number than the Australian lobster PPO. The Florida 

 lobster PPO also showed a higher physiological efficiency for both 

 substrates than the Australian lobster PPO (Table 2), which could account 

 for why Florida spiny lobsters were more susceptible to melanosis than 

 Western Australian lobsters. The kinetic and molecular properties of PPO 

 from other sources are also summarized in Table 1. With DL-DOPA as 

 substrate, different Michael is constants (K ) were observed for mushroom, 



