Metabolism Information and Courses from MediaLab, Inc.

Conditions in which glucose levels in the urine are above 100 mg/dL and detectable include:diabetes mellitus and other endocrine disordersimpaired tubular reabsorption due to advanced kidney diseasepregnancy - glycosuria developing in the 3rd trimester may be due to latent diabetes mellituscentral nervous system damagepancreatic diseasedisturbances of metabolism such as, burns, infection or fractures

Under conditions of abnormal carbohydrate metabolism such as occurs in diabetes mellitus, ketones accumulate in the blood (ketonemia) and are excreted in the urine (ketonuria). The accumulation of ketone bodies is often the cause of acidosis and coma in diabetics.

Lipoproteins differ in size and density as well as in their content (what they tend to carry). They also can differ in their origination (where they are made). Another significant difference between lipoprotein molecules is the proteins they have on their surfaces. These proteins, known as apolipoproteins, are the major identifying characteristics of a lipoprotein. There are many different apolipoproteins and we are continually learning more about them. Apolipoproteins have multiple roles. One role is that these amphipathic (detergent-like) proteins increase the overall solubility of the lipid particle, helping it to dissolve in the aqueous environment of the blood. Apolipoproteins can also function as enzyme co-factors (receptor ligands) and facilitate the transfer of their lipid cargo to specific cells. Thus, the apoliproteins are the smart or working-end of the lipoprotein particle. The apolipoproteins dictate where the particles will dock and where they can bind, and in so doing the apolipoproteins regulate lipid metabolism in the body.

Adverse drug reactions are a leading cause of morbidity and mortality in the United States. Drug metabolism is a process whereby drugs are delivered to the body, distributed, metabolized and then ultimately excreted. As there can be potentially significant differences between patient drug absorption, metabolism and excretion, molecular testing allows a physician to work with a patient's individual phenotype and/or genotype to deliver an optimum pharmaceutical selection and/or dosage.

Tests for evaluating iron metabolism are generally used as initial or screening tests for hereditary hemochromatosis (HH) as they will detect the phenotypic expression of HH. These tests include serum iron (SI), transferrin (Tf) or total iron binding capacity (TIBC), serum ferritin (SF), and unsaturated iron binding capacity (UIBC).The serum ferritin assay is also used to assess the effectiveness of HH treatment.Molecular (DNA) analyses for HFE mutations are considered to be confirmatory tests for HH which may be ordered reflexively in patients with elevated iron results. Laboratories should establish their own reference intervals for assays of iron metabolism. In general, reference intervals vary by sex and by method used for the assays discussed in the following section. Typical reference intervals are included in the following sections for instructive purposes only and should not be used for evaluating actual patient data.The results of laboratory tests assessing iron metabolism should be interpreted with caution because a number of pre-analytical and physiologic factors can affect the results. Repeating elevated test results on fasting specimens is often advisable.

Because hereditary hemochromatosis (HH) is a disease of iron overload, a review of the basic principles of iron metabolism is helpful in understanding its pathophysiology. Iron is needed by all body cells and is crucial for oxygen transport, oxidative metabolism, and cell growth and proliferation. To serve these functions, iron must be bound to protein. Iron is potentially harmful when ionized or complexed to inorganic compounds. Iron must be present in amounts sufficient to carry out these normal functions, but not in excessive amounts which may be toxic.Two types of iron-containing compounds are normally found in the body: compounds that serve in metabolic or enzymatic functions and storage compounds. Hemoglobin, myoglobin, cytochromes and other proteins are involved in oxygen transport and utilization. Iron in hemoglobin comprises about 67% of total body iron, thus erythrocytes are rich in iron. Approximately 27% of iron is found in storage compounds. Myoglobin, other tissue iron, and transport iron comprise the remaining 6% of total body iron. (2)

The diagnosis of hereditary hemochromatosis (HH) is made through a combination of laboratory tests and medical evaluation of a patient's signs and symptoms. Iron overload is identified by tests that evaluate iron metabolism, while molecular assays are needed to document mutations in the HFE gene or others such as hepcidin, hemojuvelin, or transferrin receptor. Individuals with documented iron overload who exhibit signs and symptoms consistent with HH and who possess HFE or other mutations are considered to have HH. Other causes of secondary iron overload may need to be ruled out.An example of a testing algorithm is shown.

The test for transferrin (Tf) measures the concentration of the primary carrier protein for iron. Measuring total iron binding capacity (TIBC) is an indirect method of assessing transferrin and provides comparable information. The TIBC (or transferrin) are typically performed along with the SI. Taken together, these determinations are useful in the differential diagnosis of many disorders affecting iron metabolism, including hereditary hemochromatosis (HH) and iron deficiency anemia. Tf and TIBC are typically low-normal or decreased in HH and are increased in iron deficiency. Serum transferrin can be measured directly using immunochemical methods such as nephelometry and turbidimetry. TIBC is performed in a 2-step method by adding ferric iron to the specimen in sufficient quantity to completely fill all of the iron binding sites on transferrin. Excess, unbound iron is removed by adsorption with magnesium carbonate, alumina, or ion resin. The iron content of the saturated binding protein is then measured as described for SI. Serum is the specimen of choice for Tf and TIBC. TIBC is less subject than SI to day-to-day variation and other causes of variability.A typical reference interval for TIBC is 300 - 360 micrograms/dL.(2)

Unsaturated iron binding capacity (UIBC) may also be used as a marker for altered iron metabolism. UIBC represents the portion of iron binding sites on transferrin that are not occupied by iron. Therefore, a low UIBC indicates that transferrin is highly saturated with iron, a finding consistent with hereditary hemochromatosis (HH). HH may be suspected when the UIBC is less than 143 micrograms/dL, a criterion suggested by the results of one study.(5)UIBC may be a lower cost alternative to the more complex transferrin saturation (TS). UIBC and SI are both fully automated procedures that are available on widely used laboratory instruments. The TIBC can be calculated by adding UIBC and SI, resulting in a value for TIBC that can be used for determining TS: TS = SI/(SI + UIBC) X 100

Monocytes are phagocytes which remove injured and dead cells, cell fragments, microorganisms and insoluble particles from the blood and body tissues.Monocytes also secrete substances that affect the function of other cells, especially lymphocytes.They are produced in the bone marrow, and when mature are released into the peripheral blood. Although they do serve a phagocytic role in the blood, their main site of action is the body tissues.The half-life for monocytes in the peripheral blood is approximately 8 hours. Monocytes migrate into the tissues, often to sites of inflammation, where they serve their primary purpose.Here they transform into fixed or free macrophages, and continue their function as avid phagocytes.When activated, macrophages may enlarge and have enhanced metabolism.

The liver plays a major role in converting lipophilic nonpolar molecules (drug molecules) to more polar, water-soluble forms through a series of enzymatic reactions. Drug molecules can be modified by either phase I or phase ll reactions. Phase I reactions alter chemical structure by oxidation, reduction, or hydrolysis. Phase ll reactions conjugate drugs to create products that are water-soluble.

TDM provides a quantitative measure of the circulating concentration of a drug. The physician determines if the dosage of the drug needs to be adjusted based on this information.If a drug concentration is determined to be outside the therapeutic range, it may be for one of the reasons listed in the table below. Reason Discussion Noncompliance Patients may (intentionally or unintentionally) not take the drug. TDM can thus help monitor compliance. Dosing errors The dose may have been erroneous or inappropriate given the patient's condition. Malabsorption The TDM result will reveal if the drug cannot be absorbed well through the gut and an alternative route of administration will be needed. Drug interactions Many drugs interfere with the absorption or metabolism of other drugs. These interactions will be revealed by TDM. Kidney or liver disease Any pathology that affects elimination will cause an elevation in a drug level that will be unmasked by TDM. Altered protein binding Changes in serum proteins can lead to big changes in the amount of free drug in serum. Variations in the genetics of drug-metabolizing enzymes can also affect drug concentrations in the body. This is the field of pharmacogenomics that will be discussed later in the course.

When discussing PGx, we classify a person according to his/her phenotype (metabolic capacity for a given enzyme).A poor metabolizer (PM) is a person who lacks the functional enzyme and therefore exhibits decreased metabolism of drugs. This person would require lower doses of a drug that is metabolized by that enzyme. A PM who receives a standard dose is more likely to experience unwanted side effects or toxicity. A PM can also experience diminished effects with drugs that need to be metabolized to active compounds by the enzyme in question.An ultrarapid metabolizer (UM) will require a higher dose than usual since he/she will eliminate the drug more quickly. A UM may be resistant to standard treatments, and it may take some time to adjust the dosage before therapy is achieved.An intermediate metabolizer (IM) has one wild-type (normal) copy of the gene and one absent or dysfunctional copy. The IM group is very heterogeneous.A person with normal enzyme activity is referred to as an extensive metabolizer (EM). This person should respond to standard dosages of a drug. Most people are EM's. This is the population in which most dosing regimens have been worked out in clinical trials.

Can we use therapeutic drug monitoring (TDM) to assess PGx?TDM of the drug in question can also tell us a good deal about a drug's metabolism and will also take into account all the other variables at play (co-medications, diet, impaired organ function, etc.) However, unlike genotyping and probe-drug testing, therapeutic drug monitoring must be performed during therapy, not before. So, in fact, TDM is not really used to predict therapy in PGx but serves as a confirmation of PGx findings. TDM and genotyping should be considered complementary and can be used in tandem to, first, predict and then verify appropriate serum drug levels.

The first specific PGx testing application most labs are likely to encounter is that used in patients taking warfarin. Recent studies have revealed that the variations seen in patients taking the anticoagulant warfarin are due to PGx factors. The consequences of incorrect warfarin dosing are obviously serious, with inadequate doses predisposing patients to thrombosis and higher doses placing them at risk for hemorrhage. The United States' Food and Drug Administration (FDA) recently approved updated labeling for Coumadin (warfarin sold by Bristol-Myers Squibb). The new labeling suggests that physicians incorporate PGx information into warfarin-dosing regimens for patients. Manufacturers of generic warfarin products are now adding similar labeling.

CYP2D6 has received the most attention: It is estimated that about 25% of common drugs are metabolized by CYP2D6. CYP2D6 accounts for only about 1% of all CYP450 enzymes, but it is important in the metabolism of about 100 drugs. There are more than 80 genetic variants that have been described in the CYP2D6 gene. The normal, wild-type allele displays normal metabolic activity whereas some of the variant forms have enhanced or diminished activity. The variants can be grouped generally according to the resulting alterations in protein function. The groupings correlate with four major enzyme metabolic capacities (phenotypes): poor, intermediate, extensive (normal), or ultra-rapid metabolizers.

Blood may be collected into either:Red top (clot) tubes.Speckle top tubes (serum separator tube).Gray top tubes specifically designed to preserve glucose levels. Gray top tubes contain additives such as sodium fluoride or potassium oxalate, which prevent metabolism of glucose by blood cells.

Leucine crystals indicate a problem with the metabolism of the amino acid leucine. These crystals are round to oval with radiating bands going from a center point out to the periphery, often referred to as a "wagon wheel." These crystals are soluble in hot alcohol and alkali.