Metal metabolism

Metal metabolism refers to the comprehensive set of chemical transformations that metals undergo in biological systems, including their absorption, transport, storage, and excretion processes. It is a critical aspect of biochemistry and cell biology, playing a pivotal role in maintaining homeostasis within organisms. Metals, both essential and non-essential, interact with biological systems in complex ways, influencing various physiological processes and potentially leading to toxic effects when mismanaged.

Overview

Metal metabolism encompasses the pathways and mechanisms by which organisms handle various metals. Essential metals, such as iron, zinc, copper, and manganese, are vital for the function of many enzymes, the stabilization of protein and nucleic acid structures, and in the regulation of gene expression. Non-essential metals, including lead, mercury, and cadmium, can be toxic to organisms, and their metabolism focuses on minimizing exposure and facilitating excretion.

Essential Metals

Iron is crucial for oxygen transport in hemoglobin and plays a role in electron transport within mitochondria. Zinc is involved in the function of over 300 enzymes and plays a role in immune function, protein synthesis, and DNA synthesis. Copper is essential for oxidative phosphorylation, neurotransmitter synthesis, and as a cofactor for enzymes such as cytochrome c oxidase. Manganese is important for the metabolism of amino acids, cholesterol, and carbohydrates.

Non-Essential Metals

Non-essential metals can interfere with the functions of essential metals and proteins, leading to toxicity. For example, lead can mimic calcium, affecting nerve function and brain development. Mercury can bind to sulfhydryl groups in proteins, disrupting their function. Cadmium interferes with zinc-dependent enzymes and can lead to kidney damage.

Absorption and Transport

Metals are absorbed through the diet, inhalation, and skin contact. The efficiency of absorption and the pathways used can vary significantly between metals. For instance, iron is absorbed in the duodenum via the divalent metal transporter 1 (DMT1) and exported into the bloodstream by ferroportin. Copper is absorbed in the stomach and small intestine and transported to the liver bound to albumin.

Storage and Excretion

Essential metals are stored in specific forms and locations within the body to prevent toxicity. Iron is stored as ferritin in the liver, spleen, and bone marrow. Zinc is stored bound to proteins like metallothionein. Excretion pathways for metals include the urine, feces, and bile. The liver plays a crucial role in the detoxification and excretion of metals through the bile.

Regulation

The body regulates metal metabolism through a variety of mechanisms to maintain metal homeostasis. This includes the regulation of absorption, storage, and excretion processes. For example, the hepcidin hormone regulates iron metabolism by inhibiting ferroportin, thus controlling iron absorption and release.

Disorders of Metal Metabolism

Disorders of metal metabolism can lead to either deficiency or toxicity. Iron deficiency is a common cause of anemia, while Wilson's disease and Menkes disease are genetic disorders affecting copper metabolism, leading to toxicity and deficiency, respectively. Hemochromatosis is a condition of iron overload, and lead poisoning is an example of toxicity from a non-essential metal.

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