Plasma Proteins D. JAMES HENDERSON, '64
INTRODUCTION Plasma is known to contain :1 myriad of proteins, the exact nature of many of which is as yet unknown. As a starting point, however, it is convenient to classify plasma proteins operationally on the basis of electrophoretic mobility into a number of heterogeneous fractions which are designated in order of decreasing mobility as albumins, alpha-1 and alpha-2 globulins, beta-1 and beta-2 globulins, and gamma-1 and gamma-2 globulins. The gamma-1 globulin fraction is largely fibr inogen.
PROTEIN COMPONENTS OF PLASMA 1. Albumins
The proteins of the albumin fraction may be identified on the basis of more discriminating electrophoretic techniques and ultracentrifugation as the following three: a) Crystalbumin, an easily crystallizable compound containing tryptophane and cystine among its amino acids. b) Seroglycoid, which differs from crystalhumin in containing more tryptophane and less cystine, and in the presence of a carbohydrate component. c) Mucoprotein, present in traces, comprising protein linked to an acid mucopolysaccharide. The albumins are low molecular weight proteins ( 69,000) of high electrophoretic mobilities and display good buffering properties. 2. Globulins
The globulins are in general much less homogeneous than t he albumins.1 Their separation into subgroups has been facilitated by a wide variety of techniques, perhaps the most important of which are differential precipitation, electrophoresis, should be remembered that the distinction between albumins and globulins, and indeed among alpha, beta, and gamma globulins is operational, and that proteins of similar gross solubility characteristics or electrophoretic mobilities may be vastly different with respect to their other chemical and biological properties.
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.MARcH. 1961
ultracentrifugation, and immunochemical procedures. On the basis of such studies, globulins have been found to comprise a large number of protein compounds most of which can be grouped under one or more of the following beadings. a) Glycoproteins and mucoproteins These are a group of substances in which globulin protein is linked to a complex acid mucopolysaccharide, the latter in tum comprised of hexose, acetylhexosamine, and a derivative of a hexuronic acid. The term glycoprotein has rather arbitrarily been assigned to those mucoproteins containing less than 4% hexosamine. Mucoproteins migrate with the alpha-1, alpha-2, and gamma-2 globulins, but particularly with the alpha-1 subfraction. Those of the gamma-2 subfraction are almost exclusively glycoproteins. The plasma mucoproteins are often derived from the ground substance of tissue, hence variations in their concentrations are useful in presumptive diagnosis of certain pathological conditions particularly those relating to tissue damage. b) Lipoproteins Lipoproteins, consisting of a water-soluble globulin conjugated with various lipid moieties, are represented in the alpha-1, alpha-2, and beta subfractions. In general, the beta lipoproteins tend to be high in fat content and of a large molecular weight (up to 1,300,000), whereas alpha lipoproteins are