The effect of vitamin A on plasma glycoproteins

Abstract

The rate of uptake of label from radioactive D-glucosamine and D-mannose into the plasma glycoproteins was studied in vitamin A-deficient rats by comparison with the plasma of normal pair-fed controls. Preliminary studies indicated that peak incorporation (specific activity) was reached three hours after intraperitoneal injection with labelled sugar in both vitamin A-deficient and normal rat plasma. Normal-deficient pairs were injected with the same sugar, labelled with a different isotope, and their plasma was mixed based on equal amounts of protein and fractionated on DEAE-Sephadex A-50. There was a consistent decrease m radioactivity observed m what appeared to be the alpha₁ peak in vitamin A-deficiency. This depression was on the order of 30%, when normal and deficient peak totals were compared. This effect appeared with mannose and glucosamine and was of equal magnitude for both sugars. Fractionation of this peak by gel filtration showed that most of the radioactivity was associated with one glycoprotein, which was homogeneous in 5% polyacrylamide gel electrophoresis; t.he molecular weight of this glycoprotein was estimated to be on the order of 1 x 10⁶ from its behavior on Sepharose 6B. The decrease in the incorporation of label into this peak was interpretted as representing a decreased synthesis rate in vitamin A-deficiency. A shift in the position of the peaks occurred on DEAE-Sephadex in two fractionations of glucosamine-labelled plasma. The vitamin A-deficient plasma glycoproteins were eluted slightly later than those from normal plasma, indicating either a higher negativity in deficiency or a lower molecular weight. This effect was not investigated. However, its failure to be expressed during gel filtration and its reappearance in electrophoresis suggested that charge differences were responsible for this shift.