Human alpha-fetoprotein primary structure: a mass spectrometric study

Structural characteristics of alpha-fetoprotein, including N-glycosylation, metal ion and fatty acid binding sites

Alpha-fetoprotein (AFP), a serum glycoprotein, is expressed during embryonic development and the pathogenesis of liver cancer. It serves as a clinical tumor marker, function as a carcinogen, immune suppressor, and transport vehicle; but the detailed AFP structural information has not yet been reported. In this study, we used single-particle cryo-electron microscopy(cryo-EM) to analyze the structure of the recombinant AFP obtained a 3.31 Å cryo-EM structure and built an atomic model of AFP. We observed and identified certain structural features of AFP, including N-glycosylation at Asn251, four natural fatty acids bound to distinct domains, and the coordination of metal ions by residues His22, His264, His268, and Asp280. Furthermore, we compared the structural similarities and differences between AFP and human serum albumin. The elucidation of AFP's structural characteristics not only contributes to a deeper understanding of its functional mechanisms, but also provides a structural basis for developing AFP-based drug vehicles.

Non-invasive diagnosis of endometriosis: Immunologic and genetic markers

Endometriosis, a benign gynecologic and chronic inflammatory disease, is defined by the presence of endometrial tissue outside the uterus characterized mainly by pelvic pain and infertility. Because endometriosis affects approximately 10% of females, it represents a significant socioeconomic burden worldwide having tremendous impact on daily quality of life. Accurate and prompt diagnosis is crucial for the management of this debilitating disorder. Unfortunately, diagnosis is typically delayed to lack of specific symptoms and readily accessible biomarkers. Although histopathologic examination remains the current gold standard, this approach is highly invasive and not applicable for early screening. Recent work has focused on the identification of reliable biomarkers including immunologic, ie, immune cells, antibodies and cytokines, as well as genetic and biochemical markers, ie, microRNAs, lncRNAs, circulating and mitochondrial nucleic acids, along with some hormones, glycoproteins and signaling molecules. Confirmatory research studies are, however, needed to more fully establish these markers in the diagnosis, progression and staging of these endometrial lesions.

Hepatocellular Carcinoma, Alpha Fetoprotein, and Liver Allocation for Transplantation: Past, Present and Future

Hepatocellular carcinoma (HCC) is one of the leading indications for liver transplantation and has been the treatment of choice due to the oncologic benefit for patients with advanced chronic liver disease (AdvCLD) and small tumors for the last 25 years. For HCC patients undergoing liver transplantation, alpha fetoprotein (AFP) has increasingly been applied as an independent predictor for overall survival, disease free recurrence, and waitlist drop out. In addition to static AFP, newer studies evaluating the AFP dynamic response to downstaging therapy show enhanced prognostication compared to static AFP alone. While AFP has been utilized to select HCC patients for transplant, despite years of allocation policy changes, the US allocation system continues to take a uniform approach to HCC patients, without discriminating between those with favorable or unfavorable tumor biology. We aim to review the history of liver allocation for HCC in the US, the utility of AFP in liver transplantation, the implications of weaving AFP as a biomarker into policy. Based on this review, we encourage the US transplant community to revisit its HCC organ allocation model, to incorporate more precise oncologic principles for patient selection, and to adopt AFP dynamics to better stratify waitlist dropout risk.

Biomarkers in Liver Disease

Symptoms and signs of liver diseases are highly variable depending on the etiology, disease stage, and type of liver involvement. There are different types of liver diseases; causes of liver diseases may be viral, toxic, metabolic, or autoimmune. However, in some cases, liver disease can develop as a result of diseases of other organs or systems. It is almost impossible to differentiate all of these solely on the basis of clinical symptoms and findings. Furthermore, the early stages of liver disease may be completely asymptomatic, or in some cases, the disease may progress with only subtle and non-specific symptoms. Therefore, biomarkers have a critical role in screening, diagnosis, staging, and evaluation of therapeutic response to treatment in liver diseases.

Abdominal ultrasound and alpha-foetoprotein for the diagnosis of hepatocellular carcinoma in adults with chronic liver disease

BACKGROUND

Hepatocellular carcinoma (HCC) occurs mostly in people with chronic liver disease and ranks sixth in terms of global instances of cancer, and fourth in terms of cancer deaths for men. Despite that abdominal ultrasound (US) is used as an initial test to exclude the presence of focal liver lesions and serum alpha-foetoprotein (AFP) measurement may raise suspicion of HCC occurrence, further testing to confirm diagnosis as well as staging of HCC is required. Current guidelines recommend surveillance programme using US, with or without AFP, to detect HCC in high-risk populations despite the lack of clear benefits on overall survival. Assessing the diagnostic accuracy of US and AFP may clarify whether the absence of benefit in surveillance programmes could be related to under-diagnosis. Therefore, assessment of the accuracy of these two tests for diagnosing HCC in people with chronic liver disease, not included in surveillance programmes, is needed.

OBJECTIVES

Primary: the diagnostic accuracy of US and AFP, alone or in combination, for the diagnosis of HCC of any size and at any stage in adults with chronic liver disease, either in a surveillance programme or in a clinical setting. Secondary: to assess the diagnostic accuracy of abdominal US and AFP, alone or in combination, for the diagnosis of resectable HCC; to compare the diagnostic accuracy of the individual tests versus the combination of both tests; to investigate sources of heterogeneity in the results.

SEARCH METHODS

We searched the Cochrane Hepato-Biliary Group Controlled Trials Register, the Cochrane Hepato-Biliary Group Diagnostic-Test-Accuracy Studies Register, Cochrane Library, MEDLINE, Embase, LILACS, Science Citation Index Expanded, until 5 June 2020. We applied no language or document-type restrictions.

SELECTION CRITERIA

Studies assessing the diagnostic accuracy of US and AFP, independently or in combination, for the diagnosis of HCC in adults with chronic liver disease, with cross-sectional and case-control designs, using one of the acceptable reference standards, such as pathology of the explanted liver, histology of resected or biopsied focal liver lesion, or typical characteristics on computed tomography, or magnetic resonance imaging, all with a six-months follow-up.

DATA COLLECTION AND ANALYSIS

We independently screened studies, extracted data, and assessed the risk of bias and applicability concerns, using the QUADAS-2 checklist. We presented the results of sensitivity and specificity, using paired forest-plots, and tabulated the results. We used a hierarchical meta-analysis model where appropriate. We presented uncertainty of the accuracy estimates using 95% confidence intervals (CIs). We double-checked all data extractions and analyses.

MAIN RESULTS

We included 373 studies. The index-test was AFP (326 studies, 144,570 participants); US (39 studies, 18,792 participants); and a combination of AFP and US (eight studies, 5454 participants). We judged at high-risk of bias all but one study. Most studies used different reference standards, often inappropriate to exclude the presence of the target condition, and the time-interval between the index test and the reference standard was rarely defined. Most studies with AFP had a case-control design. We also had major concerns for the applicability due to the characteristics of the participants. As the primary studies with AFP used different cut-offs, we performed a meta-analysis using the hierarchical-summary-receiver-operating-characteristic model, then we carried out two meta-analyses including only studies reporting the most used cut-offs: around 20 ng/mL or 200 ng/mL. AFP cut-off 20 ng/mL: for HCC (147 studies) sensitivity 60% (95% CI 58% to 62%), specificity 84% (95% CI 82% to 86%); for resectable HCC (six studies) sensitivity 65% (95% CI 62% to 68%), specificity 80% (95% CI 59% to 91%). AFP cut-off 200 ng/mL: for HCC (56 studies) sensitivity 36% (95% CI 31% to 41%), specificity 99% (95% CI 98% to 99%); for resectable HCC (two studies) one with sensitivity 4% (95% CI 0% to 19%), specificity 100% (95% CI 96% to 100%), and one with sensitivity 8% (95% CI 3% to 18%), specificity 100% (95% CI 97% to 100%). US: for HCC (39 studies) sensitivity 72% (95% CI 63% to 79%), specificity 94% (95% CI 91% to 96%); for resectable HCC (seven studies) sensitivity 53% (95% CI 38% to 67%), specificity 96% (95% CI 94% to 97%). Combination of AFP (cut-off of 20 ng/mL) and US: for HCC (six studies) sensitivity 96% (95% CI 88% to 98%), specificity 85% (95% CI 73% to 93%); for resectable HCC (two studies) one with sensitivity 89% (95% CI 73% to 97%), specificity of 83% (95% CI 76% to 88%), and one with sensitivity 79% (95% CI 54% to 94%), specificity 87% (95% CI 79% to 94%). The observed heterogeneity in the results remains mostly unexplained, and only in part referable to different cut-offs or settings (surveillance programme compared to clinical series). The sensitivity analyses, excluding studies published as abstracts, or with case-control design, showed no variation in the results. We compared the accuracy obtained from studies with AFP (cut-off around 20 ng/mL) and US: a direct comparison in 11 studies (6674 participants) showed a higher sensitivity of US (81%, 95% CI 66% to 90%) versus AFP (64%, 95% CI 56% to 71%) with similar specificity: US 92% (95% CI 83% to 97%) versus AFP 89% (95% CI 79% to 94%). A direct comparison of six studies (5044 participants) showed a higher sensitivity (96%, 95% CI 88% to 98%) of the combination of AFP and US versus US (76%, 95% CI 56% to 89%) with similar specificity: AFP and US 85% (95% CI 73% to 92%) versus US 93% (95% CI 80% to 98%).

AUTHORS' CONCLUSIONS

In the clinical pathway for the diagnosis of HCC in adults, AFP and US, singularly or in combination, have the role of triage-tests. We found that using AFP, with 20 ng/mL as a cut-off, about 40% of HCC occurrences would be missed, and with US alone, more than a quarter. The combination of the two tests showed the highest sensitivity and less than 5% of HCC occurrences would be missed with about 15% of false-positive results. The uncertainty resulting from the poor study quality and the heterogeneity of included studies limit our ability to confidently draw conclusions based on our results.

Serum AFP levels in patients suffering from 47 different types of cancers and noncancer diseases

Alpha-fetoprotein (AFP) is a glycoprotein and belongs to the gene family of serum albumins. The serum AFP levels were found to be elevated in the sera of liver cancer patients in 1964 and were subsequently developed and used as a liver cancer biomarker. However, elevated serum AFP levels have been observed in patients suffering from other cancer and noncancer diseases. Up to date, a systematic comparison of the serum AFP levels in different diseases has not been reported. In current study, 66,682 clinical lab test results of serum AFP levels from healthy individuals and patients with 47 different types of diseases during the past 5 years were retrieved and analyzed. Based on the mean (SD), median, and p (-Log₁₀p) values, we found that patients suffering from liver, breast, esophagus, cervical, pancreatic, endometrial, gastric, lung, rectum cancers in addition to noncancer diseases cirrhosis, nephrotic syndrome, and gastritis had significantly (p<0.05) increased, whereas patients suffering from multiple myeloma, Wilms' tumor, and other 22 types of noncancer diseases had significantly decreased median serum AFP levels than that of healthy controls. Moreover, patients with liver cancer, cirrhosis, lymphoma, bone fracture, and Wilms' tumor had highest mean serum AFP levels and the biggest SD values. In summary, the increased serum AFP levels were most evident but not specific for liver cancer patients. The potential clinical use of the increased or decreased serum AFP levels for other types of cancer and noncancer diseases and the molecular mechanisms behind our current findings need to be investigated.

Development and application of a fluorescence protein microarray for detecting serum alpha-fetoprotein in patients with hepatocellular carcinoma

Objective

To develop a simple, effective, time-saving and low-cost fluorescence protein microarray method for detecting serum alpha-fetoprotein (AFP) in patients with hepatocellular carcinoma (HCC).

Method

Non-contact piezoelectric print techniques were applied to fluorescence protein microarray to reduce the cost of prey antibody. Serum samples from patients with HCC and healthy control subjects were collected and evaluated for the presence of AFP using a novel fluorescence protein microarray. To validate the fluorescence protein microarray, serum samples were tested for AFP using an enzyme-linked immunosorbent assay (ELISA).

Results

A total of 110 serum samples from patients with HCC (n = 65) and healthy control subjects (n = 45) were analysed. When the AFP cut-off value was set at 20 ng/ml, the fluorescence protein microarray had a sensitivity of 91.67% and a specificity of 93.24% for detecting serum AFP. Serum AFP quantified via fluorescence protein microarray had a similar diagnostic performance compared with ELISA in distinguishing patients with HCC from healthy control subjects (area under receiver operating characteristic curve: 0.906 for fluorescence protein microarray; 0.880 for ELISA).

Conclusion

A fluorescence protein microarray method was developed for detecting serum AFP in patients with HCC.

Perinatal transmission of hepatitis C antigens: envelope 1, envelope 2 and non-structural 4

BACKGROUND

Perinatal exposure to hepatitis C virus (HCV) antigens during pregnancy may affect the developing immune system in the fetus. We aimed to study the perinatal transmission of HCV structural and non-structural antigens.

METHODS

Sera from 402 pregnant mothers were tested for anti-HCV antibody and HCV RNA. HCV antigens were determined in sera from 101 HCV-infected mothers and their cord blood.

RESULTS

In both serum and cord blood samples, HCV NS4 (non-structural 4) at 27 kDa, E1 (envelope 1) at 38 kDa and E2 (envelope 2) at 40 kDa were identified, purified and quantified using western blotting, electroelution and ELISA. Maternal sera and neonate cord blood samples had similar detection rates for NS4 (94.1%), E1 (90.1%) and E2 (90.1%). The mean maternal serum levels (optical density, OD) of HCV NS4 (0.87 ± 0.01), E1 (0.86 ± 0.01) and E2 (0.85 ± 0.01) did not differ significantly (p > 0.05) from those of neonatal cord blood (0.83 ± 0.01, 0.87 ± 0.01 and 0.85 ± 0.01, respectively). Also, strong correlations (p < 0.0001) were shown between sera and cord blood sample levels of HCV NS4, r = 0.77; E1, r = 0.76 and E2, r = 0.80. The vertical transmission of these antigens in vaginal delivery did not differ significantly (p > 0.05) from those in caesarean section.

CONCLUSIONS

These findings indicate that vertical transmission of HCV NS4, E1 and E2 antigens was very high. Thus, exposure to these antigens may influence the developing immune responses to natural infection or future vaccination.

Alpha-fetoprotein: a renaissance

Alpha-fetoprotein (AFP) is a major mammalian embryo-specific and tumor-associated protein that is also present in small quantities in adults at normal conditions. Discovery of the phenomenon of AFP biosynthesis in carcinogenesis by G. Abelev and Yu. Tatarinov 50 years ago, in 1963, provoked intensive studies of this protein. AFPs of some mammalian species were isolated, purified and physico-chemically and immunochemically characterized. Despite the significant success in study of AFP, its three-dimensional structure, mechanisms of receptor binding along with a structure of the receptor itself and, what is the most important, its biological role in embryo- and carcinogenesis remain still obscure. Due to difficulties linked with methodological limitations, research of AFP was to some extent extinguished by the 1990 s. However, over the last decade a growing number of investigations of AFP and its usage as a tumor-specific biomarker have been observed. This was caused by the use of new technologies, primarily, computer-based and genetic engineering approaches in studying of this very important oncodevelopmental protein. Our review summarizes efforts of different scientific groups throughout the world in studying AFP for 50 years with emphasis on detailed description of recent achievements in this field.

Modeling of three dimensional structure of human alpha-fetoprotein complexed with diethylstilbestrol: docking and molecular dynamics simulation study

It has been long experimentally demonstrated that human alpha-fetoprotein (HAFP) has an ability to bind immobilized estrogens with the most efficiency for synthetic estrogen analog - diethylstilbestrol (DES). However, the question remains why the human AFP (HAFP), unlike rodent AFP, cannot bind free estrogens. Moreover, despite the fact that AFP was first discovered more than 50 years ago and is presently recognized as a "golden standard" among onco-biomarkers, its three-dimensional (3D) structure has not been experimentally solved yet. In this work using MODELLER program, we generated 3D model of HAFP on the basis of homology with human serum albumin (HSA) and Vitamin D-binding protein (VTDB) with subsequent molecular docking of DES to the model structure and molecular dynamics (MD) simulation study of the complex obtained. The model constructed has U-shaped structure in which a cavity may be distinguished. In this cavity the putative estrogen-binding site is localized. Validation by RMSD calculation and with the use of PROCHECK program showed good quality of the model and stability of extended region of four alpha-helical structures that contains putative hormone-binding residues. Data extracted from MD simulation trajectory allow proposing two types of interactions between amino acid residues of HAFP and DES molecule: (1) hydrogen bonding with involvement of residues S445, R452, and E551; (2) hydrophobic interactions with participation of L138, M448, and M548 residues. A suggestion is made that immobilization of the hormone using a long spacer provides delivery of the estrogen molecule to the binding site and, thereby, facilitates interaction between HAFP and the hormone.

Engineering of the Saccharomyces cerevisiae yeast strain with multiple chromosome-integrated genes of human alpha-fetoprotein and its high-yield secretory production, purification, structural and functional characterization

Alpha-fetoprotein (AFP) is a biological drug candidate of high medicinal potential in the treatment of autoimmune diseases, cancer, and regenerative medicine. Large-scale production of recombinant human alpha-fetoprotein (rhAFP) is desirable for structural and functional studies and applied research. In this study we cloned and expressed in the secreted form wild-type glycosylated human rhAFP and non-glycosylated mutant rhAFP(0) (N233S) in the yeast strain Saccharomyces cerevisiae with multiple chromosome-integrated synthetic human AFP genes. RhAFP and rhAFP(0) were successfully produced and purified from the culture liquids active naturally folded proteins. Elimination of the glycosylation by mutation reduced rhAFP(0) secretion about threefold as compared to the wild-type protein showing critical role of the N-linked glycan for heterologous protein folding and secretion. Structural similarity of rhAFP and rhAFP(0) with natural embryonic eAFP was confirmed by circular dichroism technique. Functional tests demonstrated similar type of tumor suppressive and immunosuppressive activity for both recombinant species rhAFP and rhAFP(0) as compared to natural eAFP. It was documented that both types of biological activities attributed to rhAFP and rhAFP(0) are due to the fast induction of apoptosis in tumor cells and mitogen-activated lymphocytes. Despite the fact that rhAFP and rhAFP(0) demonstrated slightly less effective tumor suppressive activity as compared to eAFP but rhAFP(0) had produced statistically notable increase in its ability to induce inhibition of in vitro lymphocyte proliferation as compared to the glycosylated rhAFP and eAFP. We conclude that N-linked glycosylation of rhAFP is required for efficient folding and secretion. However the presence of N-linked sugar moiety was shown to be unimportant for tumor suppressive activity but was critically important for its immunoregulative activity which demonstrates that different molecular mechanisms are involved in these two types of biological functional activities attributed to AFP.

Alphafetoprotein in the Dutch External Quality Assurance programme: a need for improvement

Background

Elevated alphafetoprotein (AFP) concentrations may result from a variety of clinical conditions, but their role as an important tumour marker has been well established. There may be differences in AFP values due to laboratories using different methods, even though most methods have been calibrated with the same international standard (WHO IS 72/225). Therefore it is important to know the analytical performance of the various methods in relation to the analytical requirements for AFP measurement.

Methods

Annually, from January 2005 to July 2010, the results were analysed from the 65–75 laboratories that took part in the AFP survey of the External Quality Assurance programme of the Foundation Quality Control Medical Laboratories (the SKML/Binding Analysis) in the Netherlands.

Results

The Elecsys/Modular (36%) and the Immulite 2000/2500 (29%) are the methods used most. The methods show, on average, up to 15% positive and 12% negative bias, compared with the all-laboratory trimmed mean. Of the laboratories using the Immulite or the Elecsys/Modular analyser, over 70% show sufficient analytical performance to meet the Fraser criterion for method imprecision. Of the laboratories using a different method, over 50% do not meet this criterion.

Conclusions

AFP immunoassays suffer from method bias, even though all methods have been calibrated with the same international standard. Some of the methods used show insufficient performance.

Purification and characterization of Alpha-Fetoprotein from the human hepatoblastoma HepG2 cell line in serum-free medium

Alpha-fetoprotein (AFP) is a tumor-associated embryonic molecule whose precise biological function remains unclear. A complete definition of the physiological activities of this oncofetal protein has been severely limited, until now, by the lack of a purification procedure appropriate to obtain pure AFP in appreciable amount. The present report describes a purification procedure extremely rapid and simple and takes advantage of the well-known fact that AFP contains copper. We have developed a single-step purification procedure by immobilized copper-chelate affinity chromatography using the culture medium from human hepatoblastoma cell line HepG2 grown in the absence of serum. This method yields AFP at high purity and high yield. Purified AFP amino acid sequence, molecular mass, carbohydrate structure, and copper content were found to be in line with previous studies. Moreover, we found that the purified AFP has superoxide dismutase activity with efficiency similar to that of the native Cu, Zn SODs at physiological pH. This result may provide further support to the idea that AFP is a bifunctional protein, acting in cellular defence against oxidative stress both as a copper buffer and as a superoxide radical scavenger.

Structural and functional mapping of alpha-fetoprotein

Alpha-fetoprotein (AFP) is a major mammalian oncofetal protein, which is also present in small quantities in adults. It is a member of the albuminoid gene superfamily, which consists of AFP, serum albumin, vitamin D binding protein, and alpha-albumin (afamin). Although physicochemical and immunological properties of AFP have been well-studied, its biological role in embryo- and carcinogenesis and in adult organisms as well as mechanisms underlying its functioning remain unclear. During the recent decades, the biological role of AFP has been evaluated by identification of its functionally important sites. Comparison of primary structure of AFP and some physiologically active proteins revealed similarity of some polypeptide regions. This has been used for prediction of AFP functions (i.e., its multifunctionality). Localization of functionally important sites followed by determination of their amino acid composition and type of biological activity has provided valuable information for structural-functional mapping of AFP. Some peptide fragments of AFP have been synthesized and tested for biological activity. This review summarizes data on structural-functional interrelationships. We also describe functionally important AFP sites found by various groups during the last decade of structural-functional mapping of AFP with experimentally confirmed and putative biologically active sites.

Purification and characterization of a recombinant version of human α-fetoprotein expressed in the milk of transgenic goats

Alpha-Fetoprotein (AFP) is a 68 kDa glycoprotein expressed at high levels by the fetal liver and yolk with transcription repressed to very low levels after birth. Transfer of fetal AFP through the placenta into the circulation of the mother is correlated with remission of rheumatoid arthritis, multiple sclerosis, and other autoimmune disorders. AFP is therefore under development as a biopharmaceutical for the treatment of autoimmune diseases. The clinical evaluation of AFP requires the production of hundreds of grams of highly purified and biologically active protein. We have produced goats that express a form of the human AFP transgene under the control of the beta-casein promoter. In this form of rhAFP, the single N-linked glycosylation site was removed by mutagenesis (N233Q). Here, we describe a purification protocol for this recombinant human (rh)AFP from the milk of these transgenic goats. A three-column procedure was developed to produce gram quantities of highly purified rhAFP. Near- and far-UV circular dichroism spectra of human umbilical cord blood AFP and rhAFP were essentially identical, suggesting that the structure is not affected by removal of the glycosylation site. Furthermore, the cell binding and pharmacokinetics of purified rhAFP were similar to human AFP isolated from cord blood. Our results demonstrate that an active form of rhAFP can be produced on industrial scale by expression in transgenic goat milk.

Biological roles of alpha-fetoprotein during pregnancy and perinatal development

The use of alpha-fetoprotein (AFP) as a serum marker in cancer actually predates its employment in the detection of congenital defects; however, the latter use of AFP as a fetal defect marker has propelled its clinical utilization. Although the serum-marker capacity of AFP has long been exploited, less is known of the biological activities of this oncofetal protein during fetal and perinatal development. In the present review, the biological activities of AFP are discussed in light of this glycoprotein's presence in various biological fluid compartments: embryonic and fetal tissues, serum, urine, and reproductive fluids. After a review of the histochemical detection of AFP in various cells and tissues during development, AFP concentrations within various biological fluids were discussed in the context of gestational age and anatomic location. Discussion follows concerning the relationships and roles of AFP in developmental events such as erthyropoiesis, histogenesis/organogenesis, and ligand binding and in developmental disorders such as hypothyroidism, folate deficiencies, and acquired immunodeficiency disorder (AIDS). Based on its association with so many types of birth defects, malformations, and congenital anomalies, AFP can be viewed as a molecular "troubleshooter" until signal transduction pathways are established during pregnancy and prenatal development. The review concludes with a discussion of the place of AFP in the rapidly expanding field of proteomics.

Alpha-fetoprotein structure and function: relevance to isoforms, epitopes, and conformational variants

Mammalian alpha-fetoprotein (AFP) is classified as a member of the albuminoid gene superfamily consisting of albumin, AFP, vitamin D (Gc) protein, and alpha-albumin. Molecular variants of AFP have long been reported in the biomedical literature. Early studies identified isoelectric pH isoforms and lectin-binding variants of AFP, which differed in their physicochemical properties, but not in amino acid composition. Genetic variants of AFP, differing in mRNA kilobase length, were later extensively described in rodent models during fetal/perinatal stages, carcinogenesis, and organ regeneration. With the advent of monoclonal antibodies in the early 1980s, multiple antigenic epitopes on native AFP were detected and categorized, culminating in the identification of six to seven major epitopes. During this period, various AFP-binding proteins and receptors were reported to inhibit certain AFP immunoreactions. Concomittantly, human and rodent AFP were cloned and the amino acid sequences of the translated proteins were divulged. Once the amino acid composition of the AFP molecule was known, enzymatic fragments could be identified and synthetic peptide segments synthesized. Following discovery of the molten globule form in 1981, the existence of transitory, intermediate forms of AFP were acknowledged and their physiological significance was realized. In the present review, the various isoforms and variants of AFP are discussed in light of their potential biological relevance.

Highly enhanced fucosylation of serum glycoproteins in patients with hepatocellular carcinoma

BACKGROUND

We recently reported that the measurement of Lens culinaris agglutinin-reactive species of alpha-fetoprotein (AFP), alpha-1-antitrypsin (AAT) and transferrin (TF) is useful for the diagnosis of hepatocellular carcinoma (HCC) and that the molecular basis for this reactivity is fucosylation at the innermost N-acetylglucosamine residue of a biantennary sugar chain. However, the precise relationship of the fucosylation of AFP, AAT and TF in patients with HCC and liver cirrhosis is not fully understood. The aim of this study is to delineate the relationship of the fucosylation between these three glycoproteins in HCC.

METHODS

Three hundred and thirty-four patients with HCC were referred to our university hospital from 1987 to 1997. An increase in serum AFP (> 20 ng/mL) was observed in 233 (69.8%) patients with HCC. From these 233 patients with AFP-producing HCC, 60 serum samples were randomly selected and used in the present study. As a reference, samples from 60 patients with liver cirrhosis, in which 30 had increased AFP, were used. Lens culinaris agglutinin (LCA)-reactive species were determined by crossed immunoaffinoelectrophoresis (CIAE). The contents of the fucosylated biantennary chain of purified AAT and TF samples were determined as pyridylamino derivatives of each oligosaccharide with high-performance liquid chromatography (HPLC).

RESULTS

There was a highly significant correlation between LCA-reactive species by CIAE and pyridyl-amino-fucosylated biantennary sugar chain by HPLC in both AAT and TF. Lens culinaris agglutinin-reactive species of AFP, AAT and TF in HCC were significantly higher than those in liver cirrhosis. A highly statistically significant positive correlation of fucosylated glycans was observed between AAT and TF in both HCC and liver cirrhosis, but not between AFP and AAT or between AFP and TF. Accordingly, the present results indicate that highly enhanced fucosylation of serum glycoproteins was found in HCC compared with liver cirrhosis and that the combination of measurements of fucosylated AFP with AAT or TF were useful for the diagnosis of HCC.

Role of alpha-fetoprotein in the diagnosis and management of hepatocellular carcinoma

Thirty-five years after its first description, alpha-fetoprotein (AFP) remains the gold standard by which other markers are judged. Serum levels above the reference range of 10 ng/mL occur in approximately 75% of cases of hepatocellular carcinoma (HCC). In individual patients, the serum AFP level behaves as if it reflects tumour mass. However, the specificity of AFP is relatively low because moderately raised levels are also found in some patients with uncomplicated chronic liver disease. Recently, tumour-specific AFP assays have been developed. These are based on the carbohydrate side-chains on the AFP molecule which exhibit characteristic differences in AFP of different origins. Monitoring response to treatment may often be more effectively carried out by serial estimation of AFP than by conventional imaging techniques. The relative specificity of AFP for HCC has also been employed to detect circulating HCC cells and to target gene therapy.

Preparation and partial structural characterization of alpha1T-glycoprotein from normal human plasma

alpha1T-glycoprotein (alpha1T) was isolated from normal human plasma in the immunochemically homogeneous state. The partial amino acid sequence and carbohydrate chains of this glycoprotein were determined. To achieve this, the carboxymethylated alpha1T was analyzed by sequencing some of the lysylendoprotease, V8 protease, tryptic, and cyanogen bromide peptides as well as the N-terminal sequence of the protein. A large number of amino acid residues (460 amino acids) was determined by chemical procedure. The peptide sequences were compared with that of other proteins. A high degree of homology was found for proteins of the albumin family. Further, human alpha-albumin, a new member of this protein family, showed an amino acid sequence identical to that of alpha1T indicating that these two proteins are very similar in amino acid sequence and composition. These proteins are closely related to alpha-fetoprotein; however, five carbohydrate chains were found on alpha1T at Asn12, Asn88, Asn362, Asn381, and Asn467 as biantennary complex type chains and the chain on Asn362 possessed a rare consensus sequence of Asn-X-Cys. Thus, alpha1T distinguishes itself by possessing five N-glycans, a finding reported here for the first time for the ALB family.

Purification and characterization of human and mouse recombinant alpha-fetoproteins expressed in Escherichia coli

Alpha-fetoprotein (AFP) is a tumor-associated embryonic molecule whose precise biological function(s) remains unclear. A more complete analysis of the physiological activities of this oncofetal protein has, until now, been severely limited by the lack of an appropriate source from which to obtain pure AFP in any sizeable quantity. In the present investigation, we obviate this problem by cloning and efficiently overexpressing mature mouse and human AFP cDNA's in Escherichia coli. For recombinant mouse AFP (rMoAFP), large segments of the coding region were excised from the preexisting plasmids pAFP1 and pAFP2, which together encompass 90% of the AFP sequence. The mouse cDNA was made complete by the addition of N- and C-terminal encoding oligonucleotides. Mouse AFP cDNA was expressed directly as a full-length molecule in vector pTrp4 or as fusion proteins in plasmids pMALc and pRX1 under the transcriptional control of trp or tac promoters. Accumulation of rMoAFP was significantly increased in protease-deficient E. coli strains over nonprotease-deficient strains, > or = 10% of total cell protein. Of the gene fusion proteins examined, none offered significant advantage over the direct expression product in terms of recombinant protein stability, overall levels of synthesis, or facilitated purification. Recombinant AFP polypeptides expressed by pTrp4 were as expected, deposited in bacterial inclusion bodies. Subsequent to resolubilization/refolding, rMoAFP was first enriched by passage over Q-Sepharose resin followed by final purification using immobilized copper-chelate affinity chromatography. Protein sequencing of the N-terminus revealed that purified rMoAFP had a deletion of the first nine amino acids coded for by the full-length mouse AFP cDNA. Similar N-terminal deletions are observed with AFP isolates originating from natural sources. A complete human AFP cDNA was generated from a fetal liver cDNA library and was cloned into vector pTrp4. Recombinant human AFP (rHuAFP) was expressed under the identical conditions employed for rMoAFP but purification had to be modified to include preparative Mono Q anion exchange chromatography. N-terminal sequencing, amino acid compositional analysis, and electrospray mass spectrometry revealed that purified rHuAFP was intact and unaltered and that the initiator methionine was completely removed. The biological activity of recombinant AFP, as judged by its inhibitory effects on in vitro lymphocyte proliferation, was equivalent to that of the native protein. The availability of large quantities of mouse and human recombinant AFP molecules should now permit detailed structure-function analyses of this important oncofetal protein to proceed in a manner unimpeded by previous limitations in both quantity and quality of the native proteins.

Carbohydrate-based measurements on alpha-fetoprotein in the early diagnosis of hepatocellular carcinoma

Serum alpha-fetoprotein (AFP) is a useful marker for the diagnosis of hepatocellular carcinoma (HCC), although this protein also increases moderately in benign liver diseases. The serum concentration of AFP in HCC at the time of initial diagnosis is now lower than before because of advancements in techniques for imaging the liver. The AFP concentration alone cannot distinguish between HCC and benign liver diseases, especially when it is less than 1000 ng ml-1. These circumstances lead to the need to discriminate between these diseases. This has been achieved by determining the carbohydrate structures of AFP by its reactivity with Lens culinaris agglutinin (LCA). The percentage of LCA-reactive species of AFP is significantly higher in HCC than in benign liver diseases. The fucosylation of the sugar chain at the innermost N-acetylglucosamine is the molecular basis of this variation. Therefore, the term 'fucosylation index' has been introduced to express the percentage of LCA-reactive species of AFP. This index is useful for the diagnosis of HCC even if the carcinoma is at an early stage. Furthermore, it can predict the development of HCC in the follow-up of chronic liver diseases. Thus, the qualitative and quantitative measurements of carbohydrate in AFP provide us with very valuable information for the differential diagnosis of various liver diseases.

Peptide mapping of recombinant human parathyroid hormone by enzymatic digestion and subsequent fast-atom bombardment mass spectrometry

Peptide maps of recombinant human parathyroid hormone (rhPTH) were determined by both trypsin and V-8 protease digestion with subsequent fast-atom bombardment mass spectrometry (FAB-MS). Coverage of the sequence was 85% when using trypsin and 90% when using V-8 protease. Five rhPTH variants that were recombinantly produced as models of Asn deamidated type degradation products were measured, and molecular weight differences between their respective deamidated peptide fragments were completely detected. In the V-8 protease digests of some variants, characteristic peptide ions caused by the deamidation were observed and this greatly facilitated the assignment and recognition of the deamidated position. Our data suggest that FAB-mapping of rhPTH via the protease digestion methods used, appears to have great potential for structural investigations of the peptide.

Post-translational modifications in aspartate aminotransferase from Sulfolobus solfataricus. Detection of N-epsilon-methyllysines by mass spectrometry

Advanced mass spectrometric procedures have been extensively used to provide an accurate structural characterization of aspartate aminotransferase from Sulfolobus solfataricus. The amino acid sequence of this enzyme had previously been deduced from the DNA sequence. The accurate molecular mass of the protein, determined using electrospray mass spectrometry, demonstrated that the amino acid sequence deduced was correct and ruled out the possible presence of large covalent modifications which had been postulated to fit the much higher molecular mass obtained from previous SDS/PAGE experiments. The definition of the entire primary structure of aspartate aminotransferase from S. solfataricus was achieved by exploiting a new mass spectrometric mapping strategy. Initially, the molecular mass of relatively large protein fragments produced by CNBr hydrolysis was accurately determined using electrospray mass spectrometry. The protein regions where structural modifications had occurred were easily identified from their anomalous mass values. The corresponding CNBr fragments were then subdigested with suitable proteases and the resulting peptide mixtures were analysed by fast-atom-bombardment mass spectrometry. This mapping approach led to the detection of two partially modified lysine residues at positions 202 and 384, which had been converted to their N-epsilon-methyl derivatives to a substoichiometric extent.

Rapid validation of molecular structures of biological samples by electrospray-mass spectrometry

A short account is presented of the method of measuring molecular masses (M(r)) of pure biological samples by electrospray ionisation mass spectrometry. It is demonstrated that the technique yields M(r) values with an effective accuracy equal to or better than 0.008% of the calculated M(r), provided that the correct molecular structure is employed in the calculation. It is therefore recommended that this method of measuring M(r)'s should be considered to form an essential part of all studies aimed at elucidating the molecular structure of purified biological macromolecules or for confirming the identity of labelled samples of such molecules.

Factors affecting the fast atom bombardment mass spectrometric analysis of proteolytic digests of proteins

The fast atom bombardment (FAB) mass spectrometric analysis of proteolytic digests of proteins is currently used in protein structural characterization. The major current limitation of this procedure is that not all the peptides generated by enzyme digestion can be observed in the spectra. Previous studies showed that in a mixture the more hydrophilic peptides are suppressed. Several enzymatic digests of 18 different proteins ranging from 10 kDa to 67 kDa in molecular weight were examined using FAB mass spectrometry. It was observed that, even though the hydrophobicity of peptides is a factor in determining their presence or absence in the spectra, the predictions of whether or not a peptide would be detected based on this criterion varies in a wide range of values. Moreover, present results seem to indicate that the presence of particular amino acid side chains within a peptide sequence capable of forming hydrogen bonds with the matrix heavily affects the behaviour of that peptide in the mixture, despite the overall hydrophobicity of the peptide itself.