Dialysis Membrane Manipulation for Endotoxin Removal

Abstract

In the dialysis clinic, water is an essential vehicle to deliver life-saving treatment to patients suffering from varying degrees of kidney failure, both acute and chronic. Clean water is vital, as the key ingredient used to prepare hemodialysis fluid (dialysate solution), and online generation of substitution fluid for hemodiafiltration. Generally all fluids used to treat patients suffering from kidney failure may come into contact with the blood of the patient, whether directly or indirectly (across a membrane), and theoretically could transport contaminants resulting in a negative impact on patient health. Of the microbiological contaminants found in water, endotoxin is given considerable attention, given its difficulty for removal and inactivation from water and water distribution systems (Smeets et al., 2003; Perez-Garcia & Rodriguez-Benitez, 2000) and its inherent pyrogenicity (G. Lonnemann, 2000). Endotoxins are found in all gram-negative bacteria, although slight differences in chemical structure are found between varying bacterial strains. The term endotoxin is typically used to describe a complex of protein and lipopolysaccharide (LPS) molecules found in the outer cell wall of gram-negative bacteria, that either slough off during growth, or are released upon cell lysis. Endotoxin and lipopolysaccharide are typically used interchangeably in literature, although in clinical discussion the term endotoxin is most often used, as it is the metric used to monitor water and dialysis fluid quality. Lipopolysaccharide is a vital component of the outer membrane of gram-negative bacteria, providing numerous physiological functions and comprising nearly 75% of the bacterium outer surface area (Raetz, 1991). Lipopolysaccharides consist of three components: a long heteropolysaccharide chain (O-specific chain) which represents a surface antigen; a core oligosaccharide; and a lipid component termed lipid A used as an anchor in the outer cell membrane (Rietschel et al., 1994; Gorbet & Sefton, 2005). Molecular weights of most lipopolysaccharides are 10 – 20 kDa; however, due to their amphiphilic nature, LPS molecules can form aggregates (100 – 1000 kDa) which are too large to pass through dialysis membranes. It has been shown that components of lipopolysaccharide (lipid A) are able to pass through dialysis membranes, can elicit a pyrogenic response (Naveh-Many et al., 1999), and contribute to long-term morbidity and inflammation (H. Schiffl, 2000; Raj et al., 2009). Lipid A is the most conserved component of lipopolysaccharide throughout all gramnegative bacteria, and as such is responsible for the majority of the pyrogenic activity. Lipid A consists of a phosphorylated N-acetylglucosamine (NAG) dimmer connected to saturated