A novel view of albumin-supported sperm capacitation: role of lipid transfer protein-I.

OBJECTIVE

To determine if one mechanism of albumin-mediated support of human sperm capacitation is lipid (cholesterol) transfer activity and contamination of albumin with Lipid Transfer Protein-I (LTP-I).

DESIGN AND MAIN OUTCOME MEASURES

Measure lipid transfer activity in various bovine and human albumin preparations, relate this activity to albumin-supported capacitation (measured by zona-free hamster oocyte sperm penetration assay) and acrosome reactions; and attempt to detect LTP-I in active albumins. Remove LTP-I from albumin which supports capacitation and reassess this support. Reconstitute capacitation support by addition of purified LTP-I.

SETTING AND SUBJECTS

Healthy sperm donors with normal semen analyses were recruited by the Reproductive Biology-Andrology Laboratory in a university medical center.

RESULTS

Albumin preparations that effectively support capacitation have high levels of lipid transfer activity and of LTP-I, a protein responsible for lipid transfer activity. Preparations with lower levels of capacitation support have less lipid transfer activity. Removal of LTP-I from supportive albumin significantly reduces the capacitation support, and this is restored by purified LTP-I. Progesterone concentrations in these preparations are negligible.

CONCLUSIONS

The variable abilities of albumin preparations to support in vitro sperm capacitation are largely dependent on the presence of contaminating LTP-I. The cholesterol transfer activity of this protein, which is present in human serum and follicular fluid, may be one mechanism in the process of capacitation.