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Characterization of Mg2+-ATPase activity in isolated B16 murine melanoma melanosomes
1Department of Biochemistry, All India Institute of Medical Sciences, New Delhi.
Abstract:
B16 murine melanoma melanosomes were purified using sucrose density gradient centrifugation. ATPase activity was evaluated in presence of specific ATPase inhibitors, and compared with melanosome ATP-driven proton translocating activity in the melanosome. Mg2+ dependent ATPase activity was greatly inhibited (82%) by the specific inhibitors of vaculor proton translocating ATPase; Cis-didimethylsulfoxide dichloroplatinum (II) at approximately 90 microM and bafilomycin AI at two fold higher concentrations. Less inhibition, about 30 and 45% was obtained with N, N1-dicyclohexylcarbodiimide and N-ethylmaleimide, and the maximal effect occurred in the 50-100 microM and 0.1-1.5 mM ranges, respectively. These drugs at similar concentrations also inhibited the proton pumping activity to the same extent as observed for ATPase activity and half-maximal inhibition of each activity was found at nearly similar concentrations. Carbonylcyanide p-trifluoromethoxyphenyl hydra zone (FCCP) prevented ATP from setting up a pH gradient across the melanosomal membrane but stimulated Mg2+ ATPase activity significantly. Replacement of 5 mM Mg2+ with equimolar Ca2+ brought about a 60% inhibition in divalent cation-dependent ATPase- activity, and an 85% inactivation of ATP-linked melanosomal H+ pump activity. In the presence of optimal concentrations of Ca2+ and Mg2+ ATPase activity was similar to that seen in a Mg2+ medium. In Ca2+ medium ATPase activity was inhibited by CDDP and stimulated by FCCP, however these effects were two to three fold less than those observed in Mg2+ medium. FCCP failed to stimulate ATPase activity in CDDP- supplemented medium, thus suggesting that the same ATPase activity fraction was sensitive to both CDDP and FCCP. Mg2+-ATPase activity, like the proton-pump was anion dependent. The lowest activity was recorded in F medium, and increased in the order of F < So4(2-) < CL- = Br-. These results show that the ATPase activity may be related to the melanosomal proton pump.
Insights
Melanosomal ATPase activity is closely linked to the melanosome proton pump. Specific inhibitors of the vacuolar proton translocating ATPase significantly reduced both ATPase and proton pumping activities in B16 murine melanoma melanosomes.
Area of Science:
- Biochemistry
- Cell Biology
- Melanoma Research
Background:
- Melanosomes are specialized organelles within melanocytes responsible for melanin synthesis and storage.
- The precise mechanisms regulating melanosome function, particularly ion transport, are not fully elucidated.
- Understanding melanosome ATPase activity is crucial for insights into melanosome biogenesis and function.
Purpose of the Study:
- To investigate the relationship between ATPase activity and proton translocation in purified B16 murine melanoma melanosomes.
- To identify the specific type of ATPase involved in melanosome function.
- To characterize the effect of various inhibitors and ions on melanosomal ATPase and proton pump activity.
Main Methods:
- Purification of B16 murine melanoma melanosomes using sucrose density gradient centrifugation.
- Assay of Mg2+-dependent ATPase activity in the presence of specific inhibitors (cis-didimethylsulfoxide dichloroplatinum (II), bafilomycin AI, N,N'-dicyclohexylcarbodiimide, N-ethylmaleimide).
- Measurement of ATP-driven proton translocating activity and comparison with ATPase inhibition patterns.
Main Results:
- Specific inhibitors of vacuolar proton translocating ATPase (V-ATPase) significantly inhibited both Mg2+-ATPase activity (82%) and proton pumping activity.
- N,N'-dicyclohexylcarbodiimide and N-ethylmaleimide showed less inhibition of ATPase and proton pump activities.
- The protonophore FCCP stimulated Mg2+-ATPase activity but inhibited proton pumping, suggesting a coupled mechanism. Ca2+ partially inhibited ATPase and proton pump activity.
- Anion dependency of Mg2+-ATPase activity mirrored that of the proton pump.
Conclusions:
- The Mg2+-ATPase activity in B16 murine melanoma melanosomes is strongly associated with the melanosomal proton pump.
- The findings suggest that a V-ATPase may be responsible for the observed ATPase and proton translocating activities within melanosomes.
- This ATPase activity is crucial for maintaining the acidic environment within melanosomes, potentially impacting melanin synthesis and melanosome maturation.