Resumen:

El plomo es un metal pesado que por años se ha utilizado en la industria con diversos fines, por lo que tiene una amplia distribución en el ambiente. Esto, aunado a su elevada toxicidad, lo ha convertido en uno de los principales contaminantes ambientales con potencial patológico al que está expuesta la población humana. Los principales grupos de riesgo son los niños y los trabajadores de las industrias minera y metalúrgica, y de la elaboración de pinturas y el reciclaje de baterías. Otro grupo de riesgo son las familias que habitan en las áreas donde se asientan dichas industrias. El principal mecanismo tóxico del plomo es la suplantación de cationes polivalentes (esencialmente calcio y zinc) en las maquinarias moleculares del organismo, lo cual es posible gracias a una estructura iónica que le permite establecer interacciones muy favorables con los grupos que coordinan los cationes polivalentes en las proteínas, en ocasiones con más afinidad que la del propio ion suplantado. Por medio de este mecanismo afecta las proteínas transportadoras para metales, canales iónicos, proteínas de adhesión celular, diversas enzimas metabólicas y proteínas de unión al ADN, entre otros blancos moleculares. Las diferencias en la forma en que interactúa el plomo con los grupos coordinantes de la proteína con respecto a los iones nativos, pueden propiciar la adopción de conformaciones anormales en las proteínas a las cuales se une el plomo, lo que repercute directamente sobre su funcionamiento. Entre los sitios de unión para cationes polivalentes ocupados por el plomo, los de unión a calcio parecen desempeñar un papel principal en su toxicidad debido a su importancia y amplia distribución en la fisiología celular. Muchas de las alteraciones ocasionadas por el plomo se relacionan con el metabolismo celular del calcio y los distintos procesos celulares que dependen de él.

Los canales iónicos de la membrana celular representan uno de los blancos moleculares de mayor importancia patogénica para el plomo, ya que de ellos depende el funcionamiento celular coordinado y podrían ser el origen de varios de los trastornos neuropsicológicos presentes en las intoxicaciones por plomo. Este metal afecta la activación, conductancia y regulación de distintos canales iónicos de manera directa o indirecta, siendo los canales de calcio y potasio dos de los más afectados. Asimismo, el funcionamiento anormal de proteínas reguladoras intracelulares como la calmodulina, proteín cinasa C y sinamptotagminas provoca que los efectos tóxicos del plomo se extiendan a un amplio sector de la maquinaria molecular de la célula. El plomo se distribuye en el interior de la célula, por lo que afecta organelas como el retículo endoplásmico, la mitocondria y el núcleo, lo que a su vez se traduce en alteraciones en la regulación intracelular del calcio, el ensamblaje de proteínas, la generación de energía y la regulación genética, entre otras. El sistema nervioso es especialmente susceptible a la acción del plomo, en particular durante sus etapas de maduración, lo que hace que los niños sean uno de los grupos poblacionales más vulnerables a una exposición a este metal. Los estragos ocasionados ocurren aun en niveles reducidos de plomo y, aunque son irreversibles, pueden pasar inadvertidos. Entre éstos se encuentran deficiencias cognitivas, motoras y conductuales, a la vez que podría ser cofactor de trastornos neuropsicológicos más complejos como la esquizofrenia. Entre los daños producidos en el Sistema Nervioso se encuentran la excitotoxicidad, la interferencia con la neurotransmisión y la señalización intracelular en distintos niveles, y daños peroxidativos en lípidos y proteínas. Esta revisión se centra en algunos de los mecanismos moleculares implicados en la toxicidad del plomo y sus repercusiones en la fisiología celular, particularmente en lo relacionado con la excitabilidad celular.

Palabras clave: Plomo; neurotoxicidad; sitios de unión a cationes; canales de calcio; contaminación ambiental

Abstract:

Lead, a heavy metal, has been used by humans for many technological aims, a fact that has determined its actual widespread distribution. Although various actions have been taken to diminish the use and distribution of lead in the environment, it remains a significant health problem. The evolution of technological processes applied in the industry has followed economic interest. Its only in recent times that criteria related to health and ecology have been considered while designing new industries. Particularly susceptible groups are children and workers involved in mining, metallurgy, paint manufacturing and battery recycling. The communities living in areas where those industries are settled have also a higher lead exposure risk. Its high biological toxicity has determined lead to become one of the most significant environmental contaminants with pathogenic potential for humans.

The toxic mechanism of lead is essentially due to its capability to substitute other polyvalent cations (particularly divalent cations such as calcium and zinc) into the molecular machinery of living organisms. Thanks to its ionic structure, lead establishes very favorable interactions, usually with higher affinity, with chemical groups that normally coordinate divalent cations in proteins. The coordination of cations in proteins is usually achieved by negatively charged acidic residues. These residues establish ionic interactions with the positively charged ion, resulting in a change in the structure and electric charge of the protein.

These interactions determine that lead may affect different biologically significant processes, including metal transporting proteins, ionic channels, cell adhesion molecules, diverse enzymes which have metallic cofactors, signaling molecules such as calmodulin and protein kinase C and DNA binding proteins, among other molecular targets.

Lead interactions with the coordinating amino acid residues in proteins may induce an abnormal conformational configuration of proteins, as compared to the conformational structure acquired when interacting with commonly active cations, thus significantly altering its functional properties in the very complex molecular machinery. Among the biologically active sites usually occupied by lead, those related to calcium seem to have the most significant pathological importance for lead toxicity due to their widespread distribution and highly significant functional relevance for the normal cell function. Two of the principal calcium binding motifs in proteins, the EF-hand motif and the C2 motif, have an intrinsic high affinity for lead. In the case of EF-hand motifs, calmodulin is one of the most remarkable targets for lead due its importance in regulating cellular processes, being activated by lead at lower concentrations than required for calcium and displaying an abnormal activity. The C2 motif is expressed mainly in calcium dependent membrane associated proteins such as protein kinase C (PKC) or synaptotagmin. The principal characteristic in these motifs is an electrical change in the protein after the calcium binding, allowing its interaction with biological membranes. In synaptotagmin, according with the electrical characteristics of lead, the interaction of the complex lead-synaptotagmin with biological membranes is similar to the interaction calcium-synaptotagmin with membranes, which is eminently electrical. Hence, the conformation of this complex is probably different to the conformation with calcium, fact evidenced by the failure of lead-synaptotagmin to interact with other proteins of the exocitic machinery. In relation to lead neurotoxicity, membrane ionic channels seem to be among the most relevant molecular targets of lead. In particular, calcium and potassium channel function may be significantly impaired by lead, affecting the activation of calcium activated potassium channels, the inactivation process of calcium channels, and the ionic conductance of calcium channels. As occurs with other heavy metals, lead is capable of blocking the calcium channel, probably at the selectivity filter. The high affinity lead binding to the acidic residues of the filter provokes a slow flux of the metal trough the channel pore, blocking the calcium conductance. The regulation of ionic channels will be significantly altered also. Calmodulin is a common calcium sensing protein for many ionic channels and its alteration by lead could affect the channel operation. Abnormal functioning of regulatory and signaling proteins such as calmodulin, protein kinase C and synaptotagmins, which normally require calcium for its activity, may also display an abnormal functioning, thus determining a widespread metabolic influence of lead poisoning.

Lead distributes evenly into the cell thus reaching intracellular organelles, including the endoplasmic reticulum, mitochondria and the cell nucleus. This results in significant alterations of intracellular calcium metabolism and regulation due in part to the malfunctioning of calcium channels and ionic pumps in plasma membrane, endoplasmic reticulum and mitochondria. Inadequate energy generation due to mitochondrial damage and malfunctioning in cation dependent enzymes, alterations in protein folding due to the direct binding of lead to calcium activated reticular chaperones, or indirectly, altering the intrareticular calcium levels, and the disruption of the structure of DNA binding motifs such as zinc fingers, among others, promotes alterations in gene expression and DNA reparation.

Lead poisoning is one of the most important chronic environmental illnesses affecting children in modern life. Developing central nervous system is particularly susceptible to lead toxicity. At critical times in development, lead may have a disorganizing influence with long-lasting effects which may continue into teenage and beyond. Mechanisms originating this disorganizing influence in the central nervous system are a consequence of the interaction of lead with various targets as previously described; alterations of cell molecular machinery, at the systems level induce excitotoxic phenomena, interferes with neurotransmission at neurotransmitter synthesis, release and receptor activation levels, alters intracellular signaling and produce cell membrane peroxidative damage. Compared to adult lead poisoning, pediatric lead is most common and its effects may occur at reduced blood levels with subclinical symptoms; thus a high index of suspicion is necessary for physicians when dealing with pediatric patients. Long-term effects of lead may produce cognitive and motor impairment, with behavioral alterations. The particular vulnerability of the immature nervous system to the lead poisoning is probably due to the fact that in this stage of development the establishment of appropriate neural networks is highly dependent on the synaptic activity, which in turn could be altered by lead.

Lead poisoning has been considered as a potential co-factor in complex neuropsychological alterations such as schizophrenia. In this sense it is worth to note the possibility that the physical and psychic symptoms of Vincent Van Gogh may have been due to chronic lead poisoning. The following are among the clinical symptoms described by Van Gogh in his autographed letters: initial debilitation, stomatitis with loss of teeth, recurring abdominal pains, anemia (with a "plumbic" skin tone), neuropathy of the radial and saturnine encephalopathy, including epileptic crises, progressive character changes and periods of delirium, all of which meet present criteria for diagnosis of Organic Mental Disorder due to cerebral lesion or somatic illness, and Organic Character Disorder (DSM-IV-R). Apha-thujone, found in absinthe and in many popular herbal medicines, may also have contributed to Van Gogh symptoms since he was a well-known absynthe drinker.

Many countries, including Mexico, have implemented politics aimed to eliminate lead from the majority of their industrial processes. This has been carried out with considerable effort, and in some cases, with open confrontations between the scientific community and industrial sector. Although there have been actually significant advances to eliminate lead from many products (gasoline, painting manufacturing, etc.), lead is not degradable, thence once it is released in the environment it remains there for long periods of time. This implies that we should have to deal with lead poisoning in the years to come and to be aware of this diagnostic possibility in any suspicious case.

This review is centered in the description of the molecular mechanisms of lead toxicity and its repercussion in the cellular excitability and central nervous system function.

Key words: Lead; neurotoxicity; calcium-binding sites; ionic channels; calcium channels; neurotransmission