Transferrin (siderophilin) is a plasma globular protein, synthesized in the liver and to a small extent in the reticuloendothelial system and in endocrine glands (such as testes and ovaries), that carries iron through the bloodstream to the bone marrow, as well as to the liver and spleen. It has a half life of approximately 7 days.
The structure of the molecule consists of two carbohydrate residues to which six negatively charged sialic acid moieties (monosaccharide carbohydrate) can be attached and it has two N-linked polysaccharide chains.
At least 38 genetic variants of transferrin have been identified, resulting from variation in the polypeptide chain, differences in iron content, and differences in the structure of N-linked glycan chains. In healthy subjects, the tetrasialo variant is most prevalent. The biological significance of the variants is not known except for the rare congenital defect, atransferrinemia. Differing amounts of sialic acid residues influence the electrophoretic mobility of the Tf molecule and thus permit analytic separation of the different isoforms.
Plasma levels of transferrin are regulated primarily by availability of iron, because in iron deficiency states plasma levels rise and, on successful treatment with iron, return to normal levels.
Carbohydrate-deficient transferrin (CDT) is a collective term referring to isoforms of transferrin, which are deficient in sialic acid residues
Asialotransferrin—An Alternative to Carbohydrate-deficient Transferrin 2003
Before the secretion of plasma glycoproteins like transferrin, these proteins undergo a complex post translational process.
After incorporation of the mannoserich oligosaccharide core to the polypeptide in the rough endoplasmic reticulum followed by trimming, the final glycosylation occurs in the Golgi complex.
Protein glycosylation is involved in important functions such as
- intra- and inter cellular signaling,
- compartmental translocation,
- protein solubility,
- biological half-life
- enzyme activity.
This process includes sequential addition of N-acetylglucosamine, galactose, and sialic acid to the two carbohydrate units of the protein and involves at least five different glycosyltransferase:
- N-acetylglucosaminyl transferase I,
- N-acetylglucosaminyl transferase II
- N-acetylglucosaminyl transferase IV
- galactosyl- transferase
- sialyl- transferase.
After glycosylation, transferrin is secreted by exocytosis.
Much of normal catabolism is through loss of sialic acid and removal of the desialylated protein by the asialoglycoprotein receptors of the hepatic parenchymal cells obtaining CDT.
CONGENITAL DISORDERS OF GLYCOSYLATION
CONGENITAL DISORDER OF GLYCOSYLATION, TYPE Ia; CDG1A 1991
Congenital disorders of glycosylation (CDG) are a group of disorders of abnormal glycosylation of N-linked oligosaccharides caused by deficiency in 21 different enzymes in the N-linked oligosaccharide synthetic pathway .
Most commonly, the disorders begin in infancy; manifestations range from severe developmental delay and hypotonia with multiple organ system involvement to hypoglycemia and protein-losing enteropathy with normal development.
However, most types have been described in only a few individuals, and understanding of the phenotypes is limited.
In CDG-Ia, the most common form reported, the clinical presentation and course are highly variable, from death in infancy to mildly involved adults.
The diagnostic test for all types of CDG is transferrin isoform analysis to determine the number of sialylated N-linked oligosaccharide residues linked to serum transferrin.
The CDGs are inherited in an autosomal recessive manner. At conception, the theoretical risk to each sib of an affected individual is a 25% risk of being affected, a 50% risk of being an asymptomatic carrier, and a 25% risk of being unaffected and not a carrier; however, based on outcomes of at-risk pregnancies, the risk of an affected child is closer to one in three rather than the expected one in four.
Congenital disorders of glycosylation caused by defects in mannose addition during N-linked oligosaccharide assembly 2000
CDT AS A MARKER OF ALCOHOL ABUSE
Biological markers for increased risk of alcoholism and for quantitation of alcohol consumption 1990
Determination of Carbohydrate Deficient Transferrin (CDT) performed at the Laboratory of Forensic Toxicology of the University of Brescia: results from a 2005 case study. 2007
CDT if used with other tests, such as:
- gamma glutamyl transferase (GGT)
- aspartate aminotransferase (AST)
- alanine aminotransferase (ALT)
is a useful marker in screening for alcohol abuse and monitoring progress of alcoholics in treatment. In persons who consume significant quantities of alcohol (usually more than 4 or 5 alcoholic beverages a day for two weeks or more), the proportion of transferrin with zero, one, or two sialic acid chains is increased.
However it is important to understand that advanced chronic liver disease (primary biliary cirrhosis, chronic active hepatitis and drug induced hepatic insufficiency) can cause false positive results.
CDT tends to have high sensitivity (*82%*) and specificity (*97%*) in distinguishing chronic, heavy drinking subjects from abstainers or very light social drinkers. The accuracy for this marker is generally poorer among drinkers with a low level of alcohol consumption, younger alcoholics, and women.
Interesting is that the sexes seem to differ in which isoform types are increased by alcohol. It is also reported that in males CDT seems to respond mainly to frequency of drinking, whereas in females CDT is related primarily to drinking intensity with higher levels. It has been reported that the response of CDT to certain alcohol intake varies considerably between individual. Furthermore, some alcoholic patients even with very heavy daily intake fail to manifest elevated CDT level.
Furthermore transferrin is a steroid responsive protein: absolute serum CDT concentrations from healthy women typically are higher than those of healthy men, but the exact reason is unclear. In fact, disturbances of Transferrin isoforms have been observed in women as a function of hormonal status. Mean CDT values are significantly higher in the third trimester of pregnancy compared with the first and second trimesters. Carbohydrate-Deficient Transferrin, γ-Glutamyltransferase, and Macrocytic Volume as Biomarkers of Alcohol Problems in Women 2000
Similarly, week of pregnancy correlates with total Tf levels. Use of contraceptives and hormone replacement therapy also has been associated with alterations of the normal values of CDT. Interestingly, women who are entering menopause produce significantly lower levels of CDT than do premenopausal women. Such sex-based hormonal variations might contribute to the lower sensitivity of CDT as a marker of alcohol abuse among women.
The pathomechanisms for the increase in CDT isoforms during chronic alcohol abuse are not completely understood.
Alcohol could interfere with a number of glycoconjugation reactions, perhaps as the result of acetaldehyde inhibition of hepatic glycotransferases. The isoforms of transferrin have been studied as markers of alcohol consumption, as excessive ethanol
consumption results in the appearance in serum of isoforms that are carbohydrate deficient.
Many enzymes have been reported to be affected by ethanol intake. Some of them also are implicated directly in the protein glycosylation process. More specifically, the change (induction/inhibition) of two enzymes sialyl transferase and plasma sialidase activities involved in the protein sialylation/desialylation pathway. After chronic alcohol intake, liver membrane sialyl transferase decreases and plasma sialidase increases.
CDT ASSAY METHODS
Several CDT assay methods appeared promising, in particular:
- liquid chromatography
- isoelectric focusing
The isoelectric point of a transferrin isoform varies according to the number
of sialic acid group present. Moreover, quantitative evaluation of transferrin fractions is complicated.
Tf isoforms are also analysed by capillary electrophoresis and capillary zone electrophoresis.
CDT is measured by taking a sample of a patient's blood. Apparently healthy individuals with no or low reported alcohol consumption and a negative Alcohol Use Disorder Identification Test will have a %CDT <3.0 (95th percentile for the social drinking population).
Elevated levels of CDT suggest recent alcohol abuse, especially if other liver-associated enzymes are elevate.