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Alginate, a polysaccharide extracted from brown seaweed, is widely used for the microencapsulation of islets of Langerhans, allowing their transplantation without immunosuppression. This natural polymer is known to be largely contaminated. The implantation of islets encapsulated using unpurified alginate leads to the development of fibrotic cell overgrowth around the microcapsules and normalization of the blood glucose is restricted to a very short period if it is achieved at all. Several research groups have developed their own purification method and obtained relatively good results. No comparative evaluation of the efficiencies of these methods has been published. We conducted an evaluative study of five different alginate preparations: a pharmaceutical-grade alginate in its raw state, the same alginate after purification according to three different published methods, and a commercially available purified alginate. The results showed that all purification methods reduced the amounts of known contaminants, that is, polyphenols, endotoxins, and proteins, although with varying efficiencies. Increased viscosity of alginate solutions was observed after purification of the alginates. Despite a general efficiency in decreasing contamination levels, all of the purified alginates contained relatively high residual amounts of protein contaminants. Because proteins may be immunogenic, these residual proteins may have a role in persisting microcapsule immunogenicity.

Over the past 60 years, egg yolk (EY) has been routinely used in both liquid semen extenders and those used to cryopreserve sperm. However, the mechanism by which EY protects sperm during liquid storage or from freezing damage is unknown. Bovine seminal plasma contains a family of proteins designated BSP-A1/-A2, BSP-A3, and BSP-30-kDa (collectively called BSP proteins). These proteins are secretory products of seminal vesicles that are acquired by sperm at ejaculation, modifying the sperm membrane by inducing cholesterol efflux. Because cholesterol efflux is time and concentration dependent, continuous exposure to seminal plasma (SP) that contains BSP proteins may be detrimental to the sperm membrane, which may adversely affect the ability of sperm to be preserved. In this article, we show that the BSP proteins bind to the low-density fraction (LDF), a lipoprotein component of the EY extender. The binding is rapid, specific, saturable, and stable even after freeze-thawing of semen. Furthermore, LDF has a very high capacity for BSP protein binding. The binding of BSP proteins to LDF may prevent their detrimental effect on sperm membrane, and this may be crucial for sperm storage. Thus, we propose that the sequestration of BSP proteins of SP by LDF may represent the major mechanism of sperm protection by EY.

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