| OBJECTIVES:
Phenoloxidase (PO) is a key enzyme in insect cuticle sclerotization and melaninzation, which occupies several major roles in insect development and immunity. Arrest or even delay of these processes has devastating effects on insect viability, which has highlighted the need for the study of inhibitors of PO. Identification of PO inhibitors from natural sources, especially plants, has great appeal. Overall project objectives are: 1) systematic study of our natural product library to determine which compounds inhibit insect PO, followed by optimization of these lead compounds. 2) investigation of their inhibition mechanisms via structure-activity relationship study based on kinetic and mode of action data. 3) in-vivo studies of active in vitro compounds. Ingestion, contact and fumigation application feasibility and toxicity of compounds towards whole insects will be studied.
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CRIS NUMBER: 0187174
SUBFILE: CRIS
PROJECT NUMBER: CA-B*-INS-6828-H
SPONSOR AGENCY: CSREES
PROJECT TYPE: HATCH
PROJECT STATUS: TERMINATED
MULTI-STATE PROJECT NUMBER: (N/A)
START DATE: Oct 1, 2000
TERMINATION DATE: Sep 30, 2005
GRANT PROGRAM: (N/A)
GRANT PROGRAM AREA: (N/A)
CLASSIFICATION HEADINGS
KA211 - Insects, Mites, and Other Arthropods Affecting Plants
S3110 - Insects
F1000 - Biochemistry and biophysics
G4.2 - Reduce Number and Severity of Pest and Disease Outbreaks
RESEARCH EFFORT CATEGORIES
| BASIC |
100% |
| APPLIED |
(N/A)% |
| DEVELOPMENTAL |
(N/A)% |
KEYWORDS: insect biochemistry; phenoloxidases; enzyme inhibitors; inhibition; biochemical mechanisms; insect control; cuticle; melanization; insect development; immunology; plant chemistry; optimization; natural substances; mode of action; kinetics; enzyme activity; enzyme structure; biological activity; product evaluation; bioassays; analogs; enzyme function
PROGRESS: Oct 1, 2000 TO Sep 30, 2005
To study the effects of the phenoloxidase (PO) enzyme on insect cuticle sclerotization and melaninzation, we conducted enzyme inhibition assays using the compounds previously characterized as mushroom tyrosinase (the same as PO) inhibitors. Tannic acid isolated from Gallae Rhois inhibited the oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA) catalyzed by tyrosinase; it also inhibited hydroxylation of L-tyrosine. Quercetin, a common flavonol, inhibits tyrosinase by a slow and reversible reaction with residual enzyme activity. Arbutin was found to inhibit the oxidation of L-tyrosine catalyzed by mushroom tyrosinase. However, arbutin was oxidized as a monophenol substrate as soon as catalytic amounts of L-3,4-dihydroxyphenylalanine (L-DOPA) became available as a cofactor. Coumaric acid and its methyl ester were found to inhibit the oxidation of L-tyrosine catalyzed by mushroom tyrosinase. However, both were oxidized as monophenol substrates at an initially extremely slow
rate. This oxidation was significantly accelerated as soon as catalytic amounts of L-DOPA became available as a cofactor. Fisetin, which lacks the C-5 hydroxy group, was also tested; its oxidation was faster than that of quercetin. The kinetic course of the inactivation of mushroom tyrosinase by cetylpyridinium chloride (CPC) was investigated. The enzyme was inactivated by a complex scheme that has not been previously identified. We began to suspect that the standard spectrophotometric method was not be long enough to fully evaluate the phytochemicals characterized as tyrosinase inhibitors. Enzyme activity is usually monitored by measuring dopachrome formation at 475 nm accompanying the oxidation of L-3,4-dihydroxyphenylalanine (L-DOPA). Because dopachrome is gradually oxidized even further, inhibition kinetics are usually based on the data obtained within 1 min. For example, kojic acid, a well known potent tyrosinase inhibitor, did not inhibit the tyrosinase catalyzed oxidation of
L-DOPA if a longer reaction time was observed. This prompted us to reinvestigate the effect of the tyrosinase inhibitors previously characterized. Since tyrosinase is known to catalyze a reaction between two substrates, we monitored enzyme activity by measuring oxygen consumption. The polarographic method is linked to consecutive spectra and HPLC analyses. We found that many tyrosinase inhibitors isolated from plants do not inhibit the tyrosinase catalyzed oxidation of L-DOPA. However, quercetin and methyl p-coumarate isolated from the fresh flower of a Mexican medicinal plant, Trixis michuacana var longifolia (Compositae), inhibit melanin formation in murine B16 melanoma cells without affecting cell growth. This inhibition of cellular melanin formation likely involves their oxidized intermediates since both compounds are oxidized as substrates catalyzed by tyrosinase. Melanin is also found in the mammalian eye and brain. Tyrosinase may play a role in neuromelanin formation in the
human brain. It may contribute to both dopamine neurotoxicity and to the neurodegeneration associated with Parkinson's disease.
IMPACT: 2000-10-01 TO 2005-09-30
The results may provide more scientifically sound and environmentally acceptable species specific insect control agents. Further study is needed to determine if tyrosinase may play a role in neuromelanin formation in the human brain.
PUBLICATION INFORMATION: 2000-10-01 TO 2005-09-30
No publications reported this period
PROJECT CONTACT INFORMATION
| NAME: |
Kubo, I. |
| PHONE: |
510-643-6303 |
| FAX: |
510-642-7428 |
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