Proline-rich protein is a glycoprotein and an acute phase reactant.

Proline-rich protein (PRP) is a plasma protein associated with lipoproteins. In an attempt to clarify the biological significance of this protein, we isolated and characterized it and studied the biological role in plasma. PRP was isolated by immunosorber column chromatography and by gel filtration and ion-exchange chromatography. The molecular weight determined by gel filtration chromatography was 352,000, that is, about 5-times larger than the weight determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (73,800), indicating pentamer formation. About 10 or 11 isoproteins (pI 5.89-6.55) were observed by isoelectric focusing gel electrophoresis. PRP contained fucose, mannose, galactose, glucosamine and sialic acid accounting for 8.0% of the dry weight. PRP also had a hydrophilic property, as determined by charge shift electrophoresis. Levels of this protein in the human serum related to triacylglycerol-rich lipoproteins. The concentration of PRP correlated to the erythrocyte sedimentation rate (ESR), the C-reactive protein (CRP) and alpha 1- and alpha 2-globulin. Sera from patients with infection and inflammation showed significantly higher PRP levels than those noted in controls. Levels of PRP rose in parallel with ESR and CRP levels following acute myocardial infarction, and the maximal level was noted on the 7th postinfarction day. The PRP levels were elevated during the active phase of pneumonia, followed normalization. These data suggest that PRP is an acute phase reactant and may be important in the metabolism of triacylglycerol-rich lipoproteins.