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A Rapid and Specific Microplate Assay for the Determination of Intra- and Extracellular Ascorbate in Cultured Cells

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Published on: April 11, 2014

ENZYMES IN ONTOGENESIS (ORTHOPTERA) : XIII. ACTIVATION OF PROTYROSINASE AND THE OXIDATION OF ASCORBIC ACID.

T H Allen¹, J H Bodine

¹Zoological Laboratory, State University of Iowa, Iowa City.

The Journal of General Physiology

|October 30, 2009

Summary

Grasshopper protyrosinase, when activated, oxidizes ascorbic acid via quinone intermediates. This reaction is dependent on tyramine or tyrosine presence and influences melanin formation pathways.

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Published on: March 18, 2012

Area of Science:

Biochemistry
Enzymology
Insect Physiology

Background:

Protyrosinase is an inactive enzyme precursor found in grasshopper eggs.
Tyrosinase enzymes are involved in melanin biosynthesis.
Ascorbic acid (Vitamin C) is a crucial antioxidant.

Purpose of the Study:

To investigate the activation of grasshopper protyrosinase.
To determine the role of activated protyrosinase in ascorbic acid oxidation.
To elucidate the relationship between protyrosinase activity, substrates, and melanin precursor formation.

Main Methods:

Enzyme activation using sodium oleate or Aerosol.
Assaying ascorbic acid oxidation by reaction products of activated protyrosinase with tyramine or tyrosine.
Dialysis to remove naturally occurring substrates from protyrosinase preparations.

Main Results:

Activated protyrosinase, in the presence of tyramine or tyrosine, oxidizes ascorbic acid to dehydroascorbic acid.
The rate of ascorbic acid oxidation is directly proportional to tyramine or tyrosine concentration.
Ascorbic acid oxidation by the tyramine-tyrosinase reaction delays the formation of an indole quinone intermediate in melanin synthesis.
A naturally occurring substrate, removable by dialysis, is necessary for protyrosinase to catalyze ascorbic acid oxidation.

Conclusions:

Protyrosinase requires activation and specific substrates (tyramine or tyrosine) to oxidize ascorbic acid.
The oxidation of ascorbic acid by activated protyrosinase influences the kinetics of melanin precursor formation.
Dialysis can remove a necessary cofactor for this enzymatic activity, highlighting the importance of natural substrates.