Introduction to Occupational Toxicology

Module 3: Toxicology - Section 1: Introduction to Occupational Toxicology

TOX 1.8: Toxicokinetics - Distribution, Biotransformation and Excretion

Distribution

Blood is the primary method of distribution with some of the toxic substances being removed by the lymph and lung macrophages. The physiological and physicochemical nature of a substance determine the manner of distribution. The lipid solubility of substances allows them to penetrate certain barriers. Sometimes toxic agents may bind to cellular proteins and as a result accumulate in tissues. Alternatively a difference in pH gradient can lead to the accumulation of toxic substances in one area as opposed to another. Substances such as DDT or dioxin can accumulate in body fat tissues and still be present in equilibrium between fat and blood several years later.

Distribution is dependent upon blood flow to an organ and ease of passage across cell membranes.

Biotransformation, Metabolism

The enzymes involved with biotransformation or metabolism occur in almost all tissues of the body, although certain tissues are much more active in metabolism than are others. In order of quantitative importance these tissues are: liver > kidney > GI tract > lung > other tissues.

The enzymes responsible for biotransformation of xenobiotics, (foreign chemical substances), are also involved in the metabolism of natural substances. These enzymes are predominantly located in membranes (e.g. the endoplasmic reticulum, or microsomes) and are also found in the cytoplasm of cells.

The Phase 1 transformations, (e.g., oxidation, reduction, hydroxylation, nitrosation, etc.) of lipid-soluble substances most often result from an action of a family of haemoprotein oxygenases called mixed function oxygenases, (MFO), or cytochrome P- 450 enzymes, ( so called because they absorb light at a wavelength of approximately 450 nanometers).

Typically Phase 1 transformations add, create or uncover chemically reactive functional groups and do not greatly affect hydrophilic nature (water solubility). These haem containing proteins require NADPH and molecular oxygen and exhibit some substrate specificity. These enzymes can be induced to become more active through the action of some other foreign compounds. Typical inducers of Phase 1 enzymes include ethyl alcohol, phenobarbitol, polyaromatic hydrocarbons, organochlorines such as DDT or dioxins, and a variety of pharmaceutical agents. In addition specific substrates typically induce the appropriate metabolic enzymes (e.g. ethyl alcohol induces alcohol dehydrogenase and aldehyde dehydrogenase).

The rate of metabolism be increased by occupational, behavioural or environmental factors.

Phase 2 biotransformations involve a group of transferases which conjugate amino acids or other groups which typically decrease chemical reactivity and increase excretability. These enzymes add a polar water-soluble, endogenous substance (e.g. glucuronic acid, glutathione) to relatively lipid-soluble substances. The water-soluble metabolites, called conjugates, are then readily excreted. The activity or amounts of these transferases can also be increased by substrates or foreign compounds. They may be measured in blood as liver enzymes or aminotransferases.

Some agents are metabolised at a high rate while others are metabolised slowly. The rate of metabolism is usually dependent on chemical structure and physical properties. In addition there is a great variability in the activity of certain metabolising enzymes between individuals - this variability appears to be under genetic control. For example there are "fast" versus "slow" acetylators. This is mediated via various isozymes of glucosyl-sulfotransferases, etc. which result in dramatic differences in the metabolic rates of certain substances, and thus risk of toxicity, between individuals.

Question: It is likely that there will eventually be a commercially viable test able to distinguish slow from fast acetylators? Should "slow" be excluded from jobs involving exposure to chemicals?

Phase 1 biotransformations of certain compounds can result in metabolites which are more/highly chemically reactive and capable of reacting with biologic tissues, i.e. more toxic. Once formed, electrophilic metabolites such as epoxides, N-hydroxy compounds, etc., can react with nucleophilic groups on cellular macromolecules (proteins and nucleic acids) and alter cell function or result in mutation. Cell death can result or genetic material be altered. The formation of reactive metabolites is the basis for some severe toxicities and the cause of chemically induced carcinogenesis.

Biotransformation then is a double-edged sword. On the one hand, it is often the major factor in determining the excretability of a substance. On the other hand, most xenobiotics must be activated to toxic metabolites. Lipid-soluble compounds would be retained for a long time in the body unless converted to more water-soluble metabolites which can be readily excreted. Metabolites may be pharmacologically or toxicologically active or they may be biologically inactive. A high degree of water solubility from Phase 2 conjugation processes usually imparts less activity because of limited distribution to tissues and rapid-excretion. Relatively lipid-soluble metabolites may be inactive because of an inability to fit receptors. For substances absorbed through the gastrointestinal tract there is a "first pass" effect whereby substances are biotransformed in the liver before they reach systemic circulation, resulting in overall lower systemic bioavailability.

Excretion

Substances can be excreted via the lungs, stool or kidney. Most polar substances are excreted via the kidney. Substances metabolized via the liver are usually excreted in the bile and then via the stool. Fat soluble substances may be excreted via breast milk.

Kidneys

Excretion of toxicants after biotransformation usually occurs though passive glomerular filtration or tubular diffusion. Alternatively active tubular secretion may take place. Active secretion requires energy expenditure,is an enzyme dependent and saturable mechanism.

The kidney receives 25% of cardiac output and 20% of the output is filtered by the glomeruli. The glomeruli have pores of 70 nm which keep all the cellular components and large molecular weight proteins such as albumin in the intravascular space. Unbound toxicants with M.M. Ł 60 000 daltons are filtered into the urinary space. Toxicants with high lipid/water partition coefficients are reabsorbed whereas ionized/polar substances are excreted. Bases are best excreted in acid urine and vice versa allowing one to hasten excretion by modifying the pH of the urine, e.g. phenobarbital (a weak acid pKa = 7.2) is best excreted after urine alkalinization.

Liver - biliary system

Substances such as lead, arsenic, manganese, DDT, are excreted in bile. As with renal tubular excretion, plasma protein bound toxins are available for biliary excretion. Organic compounds are frequently biotransformed to polar metabolites prior to biliary excretion, but intestinal microflora can hydrolyze the glucuronide or sulfate conjugates enabling reabsorption. This is termed the enterohepatic recirculation and occurs with methyl mercury and organochlorines such as DDT. Biliary excretion can be induced by phenobarbital or steroids. Adsorbents of fat or cholesterol can interrupt the enterohepatic cycle, e.g. workers exposed to chlorinated hydrocarbon pesticide may be treated with the bile acid resin cholestyramine.

Gut

The gut may be an important route of excretion of organometals especially methylmercury and organochlorines such as DDT, Dioxin and PCBs.

Lung

This is a main route of excretion for gases e.g. carbon monoxide, halothane, and to a lesser volatile organic compounds.

Subsidiary note: Mechanisms of transfer across tissues:

The kidney, Liver and CNS each has a specialized active transport mechanism for nutrient supply and for excretion of organic acids and bases.

Half-life - an important toxicokinetic concept

An important concept is that of half-life of a substance. This is relevant, for example, to biological monitoring. It is also relevant to the interpretation of disciplinary alcohol or drug monitoring undertaken by some employers.

(It is critical in forensic medicine in estimating when someone may have ingested a toxin, or how much they must have ingested, e.g. alcohol.)

If a substance is eliminated at a constant rate (so-called zero order kinetics, i.e. independent of concentration), 90% will be eliminated in 3.5 half-lives. E.g. t_˝ of nicotine = 2 hrs.