Structural glycoprotein, fact or artefact

It has been repeatedly claimed that structural glycoprotein or SGP is a major component of many connective tissues, particularly aorta. SGP is usually defined as material extractable from tissue with chaotropic agents such as urea and having an amino acid composition matching that of other, similar extracts. It has been claimed that SGP comprises 10-60% of the dry weight of aortic tissue. This paper describes a study in which aorta was exhaustively extracted with agents previously claimed to extract SGP. All the extracts yielded mixtures of proteins which, upon separation by polyacrylamide gel electrophoresis, were found to consist largely of actin, collagen, myosin and other known proteins. No significant quantity of any one unidentified protein was found which could have been SGP. This study casts grave doubt upon the existence of SGP as a major component of aorta.