Abstract: In this research, we studied the inhibitory effects of different concentrations (1, 5, 10, 15, and 20 g/L) of hypotaurine (HTU) on the activity of polyphenol oxidase (PPO) from Penaeus vannamei, and further we determined the inhibition type and inhibition constant. Meanwhile, the effects of HTU on PPO conformation were studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism (CD) spectroscopy, surface hydrophobicity measurement and endogenous fluorescence spectroscopy. The results showed that HTU could inhibit 79.38% of PPO activity when its concentration was 20 g/L, and the half-maximum inhibition concentration (IC50) value was 16.09 g/L. The Lineweaver-Burk double reciprocal plot showed that the Vmax remained unchanged while the Km increased. It was presumed that HTU was a competitive inhibitor with an inhibition constant KI of 0.445 mmol/L. SDS-PAGE results showed that the molecular mass of the enzyme did not change significantly with increasing HTU concentration. The results of CD spectroscopy and surface hydrophobicity showed that the secondary structure of the enzyme was transformed from α-helix and β-sheet to β-turn and random coil, and partial unfolding occurred, leading to the exposure of hydrophobic amino acid residues and enhanced surface hydrophobicity. HTU could quench the endogenous fluorescence of PPO and result in a red shift in the maximum emission wavelength. Therefore, it is speculated that HTU may affect the enzymatic activity by changing the spatial structure of PPO.

Key words: Penaeus vannamei; polyphenol oxidase; hypotaurine; enzymatic activity; enzyme conformation