Dr. Ricardo González Méndez is a Professor in the Department of Radiological Sciences at the University of Puerto Rico (UPR) School of Medicine, where he has been a member of the Faculty since 1989. He was the Associate Dean for Academic Affairs at the UPR Medical Sciences Campus from 1999-2002. During 2013-2014 he served as Interim Dean for Academic Affairs at the UPR Medical Sciences Campus. He is currently the President of the Concurrent Degrees Committee for the MD-PhD programs at the UPR School of Medicine.Dr. Gonzalez has a Ph.D. in Biophysics from Stanford University in California (1987), and postdoctoral studies in NMR and Anesthesia at the University of California-San Francisco (1983-1986). He received a B.S. in Biology from the University of Puerto Rico, Mayagüez Campus (1978). He is the author of over 50 publications and numerous presentations at national and international conferences. He is co-inventor on a recent patent related to imaging contrast agents.He has significant experience in bioinformatics, biophysics, medical imaging, science and policy, quantitative modeling and analysis, and risk analysis. Address: Puerto Rico
Polyunsaturated fatty acids are made in deep‐sea organisms by the activity of a polyketide syntha... more Polyunsaturated fatty acids are made in deep‐sea organisms by the activity of a polyketide synthase multienzyme. The final step in the biosynthesis of polyketides and fatty acids is catalyzed by the activity of the thioesterase (TE) domain, which cleaves the final product off of the carrier protein. However, no TE domain has been identified in any of the known PUFA synthase clusters. We propose that orf6 gene is a candidate TE, since it is conserved among other bacteria with PUFA synthase clusters and is adjacent to the PUFA synthase cluster in Photobacterium profundum. We have cloned, expressed and purified Orf6 protein with the aim of characterizing the protein structurally and functionally. The Orf6 structure, determined by X‐ray crystallography at 1.0Å resolution, demonstrates a hot‐dog fold similar to the 4‐hydroxybenzoyl‐CoA TE family. Results from enzyme assays show that Orf6 has no activity toward long‐chain fatty acids, including the expected full length product eicosapenta...
The effect of elevated temperature (44 degrees) on the intracellular uptake of the 2-nitroimidazo... more The effect of elevated temperature (44 degrees) on the intracellular uptake of the 2-nitroimidazole hypoxic cell radiosensitizer, misonidazole (MIS), and analogues more hydrophilic than MIS was studied in Chinese hamster ovary cells. It was found that the intracellular uptake of these compounds which enter cells by restricted passive diffusion can be enhanced approximately 4-fold when incubated at 44 degrees compared to the uptake at 37 degrees. Peak intracellular uptake (expressed as the ratio of intracellular concentration to extracellular concentration) following incubation of cells in 2 mM MIS was 100% at 44 degrees but only 25% at 37 degrees. Furthermore, a short-term nonlethal heat pulse (44 degrees for 15 min) with MIS present caused a 2-fold enhancement in uptake which was sustained for an additional 45 min at 37 degrees. This same nonlethal heat pulse was found to induce a similar enhancement in uptake even when MIS was added at subsequent time intervals at 37 degrees. The heat pulse induced a time-related enhancement of uptake at 37 degrees which increased for 1 hr and persisted for at least 6 hr. Finally, in vitro radiosensitization studies of hypoxic Chinese hamster ovary cells showed that the nonlethal heat pulse of 44 degrees for 15 min could greatly enhance the sensitization by low concentrations (0.5 mM) of MIS added after heating due to increased intracellular concentrations of the drug. MIS (0.5 mM) alone achieved a radiosensitization enhancement ratio of 1.29 (compared to irradiated hypoxic cells alone), while the addition of the short-term heat pulse, which had only a minor effect itself, achieved an enhancement ratio of 1.78.
The effects of hyperthermia (exposure to 41-47 degrees C) on the intracellular pH and membrane po... more The effects of hyperthermia (exposure to 41-47 degrees C) on the intracellular pH and membrane potential have been studied using Chinese hamster ovary HA-1 cells. Our goal was to determine whether intracellular pH changes or changes in membrane potential correlated with cell killing. The intracellular pH (pHi) was measured using the DMO partitioning technique. A rapid acidification of the intracellular environment was observed at all the elevated temperatures studied. The pHi reached a plateau value of approximately 6.9, and started reversing towards normal values upon prolonged exposure to heat. Similar patterns were seen for delta pH (pHi-pHo). The membrane potential difference (delta psi) was measured using the fluorescence quenching of 3,3-dipropylthio-carbocyanine, and calibrated using a 86Rb+ diffusion potential. We found that delta psi falls to zero only upon prolonged exposure to temperatures above 43 degrees C. When the external pH was changed from normal values the drop in delta psi occurred more readily. Development of thermotolerance resulted in an increase in the time required to make delta psi change by half. The changes in delta psi were shown to be irreversible. When the proton electrochemical gradient (delta mu H+) was calculated using the measured values of delta psi and delta pH, the trends observed were the same as those seen for delta psi. The changes observed for pHi can be accounted for by the changes in the pK values of the components involved in the intracellular buffering. The changes in delta psi and delta mu H+ may reflect the physical breakdown of the transmembrane H+ gradients, which may be the actual mechanical process of cell death. No correlation of cell survival with the measured parameters was observed.
Twist protein family members are expressed in different tissues during early stages of embryogene... more Twist protein family members are expressed in different tissues during early stages of embryogenesis and their presence is essential for proper development and survival. Though the highly conserved basic helix-loop-helix (bHLH) and carboxy-terminal domains of Twist proteins have received a lot of attention, the amino-terminal region and the evolution of such proteins merit further study, particularly among mammals. We analyzed the conservation of the amino-terminal region of different vertebrate Twist proteins by sequence comparisons. We identified two putative de novo motifs (SSSPVSP and SEEE), specifically in mammals. In addition, a number of amino acid substitutions present in the nuclear localization signals (NLSs) of a few species demonstrate that there could be other possible residues influencing the nuclear localization of such proteins, with the G residue of the second NLS being a potential target. By phylogenetic analysis, we investigated the origin and evolution of the two paralog twist proteins...
Twist proteins belong to the basic helix-loop-helix (bHLH) family of multifunctional transcriptio... more Twist proteins belong to the basic helix-loop-helix (bHLH) family of multifunctional transcriptional factors. ADD1, RunX2 and MEF2 have been shown to interact with TWIST2 in studies that suggest th...
Polyunsaturated fatty acids are made in deep‐sea organisms by the activity of a polyketide syntha... more Polyunsaturated fatty acids are made in deep‐sea organisms by the activity of a polyketide synthase multienzyme. The final step in the biosynthesis of polyketides and fatty acids is catalyzed by the activity of the thioesterase (TE) domain, which cleaves the final product off of the carrier protein. However, no TE domain has been identified in any of the known PUFA synthase clusters. We propose that orf6 gene is a candidate TE, since it is conserved among other bacteria with PUFA synthase clusters and is adjacent to the PUFA synthase cluster in Photobacterium profundum. We have cloned, expressed and purified Orf6 protein with the aim of characterizing the protein structurally and functionally. The Orf6 structure, determined by X‐ray crystallography at 1.0Å resolution, demonstrates a hot‐dog fold similar to the 4‐hydroxybenzoyl‐CoA TE family. Results from enzyme assays show that Orf6 has no activity toward long‐chain fatty acids, including the expected full length product eicosapenta...
The effect of elevated temperature (44 degrees) on the intracellular uptake of the 2-nitroimidazo... more The effect of elevated temperature (44 degrees) on the intracellular uptake of the 2-nitroimidazole hypoxic cell radiosensitizer, misonidazole (MIS), and analogues more hydrophilic than MIS was studied in Chinese hamster ovary cells. It was found that the intracellular uptake of these compounds which enter cells by restricted passive diffusion can be enhanced approximately 4-fold when incubated at 44 degrees compared to the uptake at 37 degrees. Peak intracellular uptake (expressed as the ratio of intracellular concentration to extracellular concentration) following incubation of cells in 2 mM MIS was 100% at 44 degrees but only 25% at 37 degrees. Furthermore, a short-term nonlethal heat pulse (44 degrees for 15 min) with MIS present caused a 2-fold enhancement in uptake which was sustained for an additional 45 min at 37 degrees. This same nonlethal heat pulse was found to induce a similar enhancement in uptake even when MIS was added at subsequent time intervals at 37 degrees. The heat pulse induced a time-related enhancement of uptake at 37 degrees which increased for 1 hr and persisted for at least 6 hr. Finally, in vitro radiosensitization studies of hypoxic Chinese hamster ovary cells showed that the nonlethal heat pulse of 44 degrees for 15 min could greatly enhance the sensitization by low concentrations (0.5 mM) of MIS added after heating due to increased intracellular concentrations of the drug. MIS (0.5 mM) alone achieved a radiosensitization enhancement ratio of 1.29 (compared to irradiated hypoxic cells alone), while the addition of the short-term heat pulse, which had only a minor effect itself, achieved an enhancement ratio of 1.78.
The effects of hyperthermia (exposure to 41-47 degrees C) on the intracellular pH and membrane po... more The effects of hyperthermia (exposure to 41-47 degrees C) on the intracellular pH and membrane potential have been studied using Chinese hamster ovary HA-1 cells. Our goal was to determine whether intracellular pH changes or changes in membrane potential correlated with cell killing. The intracellular pH (pHi) was measured using the DMO partitioning technique. A rapid acidification of the intracellular environment was observed at all the elevated temperatures studied. The pHi reached a plateau value of approximately 6.9, and started reversing towards normal values upon prolonged exposure to heat. Similar patterns were seen for delta pH (pHi-pHo). The membrane potential difference (delta psi) was measured using the fluorescence quenching of 3,3-dipropylthio-carbocyanine, and calibrated using a 86Rb+ diffusion potential. We found that delta psi falls to zero only upon prolonged exposure to temperatures above 43 degrees C. When the external pH was changed from normal values the drop in delta psi occurred more readily. Development of thermotolerance resulted in an increase in the time required to make delta psi change by half. The changes in delta psi were shown to be irreversible. When the proton electrochemical gradient (delta mu H+) was calculated using the measured values of delta psi and delta pH, the trends observed were the same as those seen for delta psi. The changes observed for pHi can be accounted for by the changes in the pK values of the components involved in the intracellular buffering. The changes in delta psi and delta mu H+ may reflect the physical breakdown of the transmembrane H+ gradients, which may be the actual mechanical process of cell death. No correlation of cell survival with the measured parameters was observed.
Twist protein family members are expressed in different tissues during early stages of embryogene... more Twist protein family members are expressed in different tissues during early stages of embryogenesis and their presence is essential for proper development and survival. Though the highly conserved basic helix-loop-helix (bHLH) and carboxy-terminal domains of Twist proteins have received a lot of attention, the amino-terminal region and the evolution of such proteins merit further study, particularly among mammals. We analyzed the conservation of the amino-terminal region of different vertebrate Twist proteins by sequence comparisons. We identified two putative de novo motifs (SSSPVSP and SEEE), specifically in mammals. In addition, a number of amino acid substitutions present in the nuclear localization signals (NLSs) of a few species demonstrate that there could be other possible residues influencing the nuclear localization of such proteins, with the G residue of the second NLS being a potential target. By phylogenetic analysis, we investigated the origin and evolution of the two paralog twist proteins...
Twist proteins belong to the basic helix-loop-helix (bHLH) family of multifunctional transcriptio... more Twist proteins belong to the basic helix-loop-helix (bHLH) family of multifunctional transcriptional factors. ADD1, RunX2 and MEF2 have been shown to interact with TWIST2 in studies that suggest th...
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Papers by Ricardo Gonzalez-Mendez