Comparison of desialylation of rat transferrin by cellular and non-cellular methods

Use of capillary zone electrophoresis for differentiating excessive from moderate alcohol consumption

BACKGROUND

The poorly sialylated transferrin isoforms in serum were analyzed by capillary zone electrophoresis (CZE) to differentiate moderate from heavy alcohol consumption.

METHODS

We enrolled 614 volunteers, classified after interviews, self-reported drinking habits, and AUDIT scores as alcohol abusers (consuming >50 g/day ethanol for the previous 3 months or longer; n = 413) or moderate drinkers (<30 g/day ethanol; n = 201). Serum transferrin isoforms were separated at 28 kV and monitored at 214 nm on a P/ACE 5500 CZE with use of fused-silica capillaries and the related CEofix CDT reagent set. Immunosubtraction by anti-human transferrin and electrophoretic migration times identified the isoforms. Previous markers of alcohol abuse and an assay combining anion-exchange minicolumn chromatography with immunoturbidimetry (%CDT) were included in the study. Sensitivities and specificities were compared by ROC analysis.

RESULTS

The asialylated isoform was missing in 95% of moderate drinkers but present in 92% of alcohol misusers. Disialotransferrin had a specificity and sensitivity of 0.75 at a cutoff of 0.7% of total transferrin, whereas the sum (asialo- + disialotransferrin) at a threshold of 1.2% had a sensitivity of 0.73 and a specificity of 0.92. Trisialotransferrin values did not distinguish between the two populations. Sensitivities and specificities of %CDT averaged 0.77 and 0.74, respectively, at a 2.6% cutoff; 0.67 and 0.83 at 2.8%; and 0.63 and 0.90 at 3%. CDT data were more sensitive and specific for males. Conventional biomarkers appeared less discriminating.

CONCLUSIONS

Asialotransferrin detected by CZE in sera of alcohol abusers offers the highest discrimination between excessive and moderate drinking.

Desialylation of human apolipoprotein E decreases its binding to human high-density lipoprotein and its ability to deliver esterified cholesterol to the liver

Apolipoprotein E (apoE) plays a significant role in the delivery of high-density lipoprotein (HDL) cholesterol to the liver via the apoB/E receptor. The roles of the apoE sialylation status in its association with HDL and in the reverse cholesterol transport (RCT) function of HDL have not been well defined. Furthermore, long-term ethanol treatment impairs apoE sialylation and leads to its decreased content in HDL. Therefore, we investigated the association of either sialo apoE (SapoE) or desialo apoE (DSapoE) with HDL and its effect on the RCT function of HDL. The dextran sulfate precipitation method showed that [125I]DSapoE binding to HDL was 27.3% (P < .02) to 35.5% (P < .001) lower versus [125I]SapoE. Scatchard analysis of the specific binding data showed that [125I]SapoE had 11.2 times more affinity for HDL than [125I]DSapoE based on size-exclusion chromatography (Kd = 89.7 v 1,010 nmol/L). Similarly, [1251]HDL had 4.5 times more affinity for SapoE compared with DSapoE based on solid-phase binding (Kd = 21.9 v 104.4 nmol/L). Furthermore, esterified cholesterol uptake from reconstituted HDL particles (rHDLs) by HepG2 cells increased over basal uptake up to 153% when rHDLs contained SapoE, versus only 37% with DSapoE. Enzymatic resialylation of DSapoE completely restored its HDL-binding and RCT properties, identical to those of SapoE. It is therefore concluded that desialylation of apoE decreases its binding to plasma HDL, leading to an impaired RCT function.

Laboratory markers of ethanol intake and abuse: a critical appraisal

Laboratory markers for ethanol intake and abuse and chronic alcoholism currently in use have been critically reviewed. The merits and pitfalls of each test have been evaluated. The clinical use of the new test of carbohydrate-deficient transferrin has been particularly emphasized. Carbohydrate-deficient transferrin currently provides the highest specificity and sensitivity of all commonly used markers of alcoholism.

Proteoglycan synthesis by cultured liver endothelium: the role of membrane-associated heparan sulfate in transferrin binding

Liver endothelium has been reported to possess membrane receptors for the iron-binding protein transferrin (Tf). Similarly, the core protein of proteoglycans (PG) associated with cell membrane in many cell systems can bind Tf. To find out if membrane-associated proteoglycans can explain Tf-binding ability of liver endothelium, we investigated the synthesis and distribution of proteoglycans by isolated, cultured liver capillary endothelium. Cells were isolated and cultured for 48 h in sulfate-free medium and pulse-labeled with 35SO4. The relative distribution of 35SO4-labeled macromolecules, determined in the extracellular (EC), membrane-associated (MA), and intracellular (IC) pools, was respectively 74, 15, and 10%. Membrane-associated proteoglycan (MA-PG) was further purified by ion exchange and gel chromatography. Glycosaminoglycan (GAG) chain characterization indicated about 78% chondroitin sulfate, 7% dermatan sulfate, and about 14% heparan sulfate (HS). Similar GAG chain characterization was made for PG in the EC and IC pools. Transferrin-binding ability of MA-PG was studied by affinity column chromatography, using CNBr-activated sepharose bound to transferrin. About 15% of the labeled MA-PG was specifically bound to Tf-affinity column and could be eluted by excess soluble Tf. This proportion was similar to the proportion of HS in the total membrane-associated pool. Moreover, the eluted labeled material was susceptible to pretreatment with heparitinase, confirming its HS nature. We conclude that the transport capillary endothelium of the liver can synthesize HS proteoglycans which are membrane-associated and this MA-HS pool can bind transferrin. The finding may provide a molecular basis for transferrin binding to liver endothelium and may explain the subsequent transendothelial transport of iron-transferrin complexes into the liver.