Structural modification of polyfunctional rubratoxin B: effects on mammalian adenosine triphosphatase
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Abstract:
Rubratoxin B, an alpha, beta unsaturated lactone containing bisanhydride metabolite of certain toxigenic strains of penicillium molds, significantly inhibited in vitro brain microsomal Na+ - K+ adenosine triphosphatase from swine, mouse, and rat with IC50's of 6.76, 6.67, and 6.80 x 10(-6) M, respectively. Mitochondrial Mg++ oligomycin-sensitive ATPase from mouse liver was also inhibited with an IC50 of 6.30 x 10(-6) M rubratoxin B. Structural modification of the polyfunctional parent compound (rubratoxin B) to rubratoxin A (a gamma lactol replaces one of the anhydride moieties) or formation of the dihydro analogs of rubratoxins A and B (2,3 saturated delta lactone) decreased inhibition to all ATPase systems. The structure-activity relationship relative to ATPase inhibition was rubratoxin B greater than dihydrorubratoxin B greater than rubratoxin A greater than dihydrorubratoxin A. The order of reactivity by these analogs to brain microsomal Na+ -K+ ATPase from 3 mammalian species and to mouse hepatic mitochondrial Mg++ ATPase was similar, indicating no significant species variations. These results suggest that the previously demonstrated in vivo inhibition of adenosine triphosphatase preparations by rubratoxin B was the result of the intact parent compound, because chemical alteration of any one of the functional moieties (either the maleic anhydride ring or conjugated lactone) resulted in significantly decreased inhibition of ATPase activity.
Insights
Rubratoxin B significantly inhibits brain and liver ATPase enzymes in mammals. Structural changes to rubratoxin B decrease its inhibitory effects, suggesting the intact molecule is responsible for in vivo toxicity.
Area of Science:
- Biochemistry
- Toxicology
- Enzymology
Background:
- Rubratoxin B is a toxic metabolite produced by Penicillium molds.
- Adenosine triphosphatase (ATPase) enzymes are crucial for cellular energy processes.
- Previous studies indicated rubratoxin B's in vivo inhibition of ATPase preparations.
Purpose of the Study:
- To investigate the in vitro inhibitory effects of rubratoxin B and its analogs on ATPase activity.
- To determine the structure-activity relationship of rubratoxin B in ATPase inhibition.
- To elucidate the role of the intact rubratoxin B molecule in its toxicological effects on ATPase.
Main Methods:
- In vitro enzyme inhibition assays using brain microsomal Na+ - K+ ATPase and mitochondrial Mg++ ATPase.
- Testing of rubratoxin B, rubratoxin A, dihydrorubratoxin A, and dihydrorubratoxin B.
- Comparative analysis of inhibition across different mammalian species (swine, mouse, rat).
Main Results:
- Rubratoxin B significantly inhibited both Na+ - K+ ATPase and Mg++ ATPase with low IC50 values.
- Structural modifications (rubratoxin A, dihydro analogs) markedly reduced ATPase inhibition.
- The order of inhibitory potency was rubratoxin B > dihydrorubratoxin B > rubratoxin A > dihydrorubratoxin A.
- No significant species variation in ATPase inhibition was observed.
Conclusions:
- The intact structure of rubratoxin B is essential for its potent inhibition of ATPase enzymes.
- Chemical alterations to the maleic anhydride ring or conjugated lactone moiety diminish inhibitory activity.
- These findings support the hypothesis that rubratoxin B acts directly on ATPase in vivo.