Exchange of lipids between spermatozoa and seminal plasma in normal and pathological human semen

A metabolomic study uncovering key amino acids and amines in Duroc boar semen as biomarkers of sexual maturity

Metabolomic analysis of boar semen associated with sexual maturation is essential for improving fertility management and breeding, with amino acids and amines playing key roles in the reproductive process. This study aimed to explore changes in amino acids and amines in boar spermatozoa and seminal plasma during puberty to sexual maturity and identify potential biomarkers of sexual maturity. Semen was collected from the same 15 Duroc boars over time at approximately 7 months (Age 1), 8.5 months (Age 2), and 10 months (Age 3). Liquid chromatography-mass spectrometry was used to analyse amino acids and amines in spermatozoa and seminal plasma separately. Multivariate analysis (PLS-DA) revealed pronounced age-dependent changes in amino acids and amines in spermatozoa between Age 1 and Age 3, and more subtle shifts in seminal plasma. Univariate analysis (Repeated measure ANOVA/Friedman) revealed that glutamate and taurine had significant pairwise differences in seminal plasma (P < 0.05). In sperm, 15 amino acids (glutamate, alanine, aspartate, choline, taurine, histidine, methionine, tryptophan, leucine, cystine, tyrosine, arginine, lysine, valine and glycine) exhibited significant pairwise differences (P < 0.05). VIP scoring (>1.5) prioritised glutamate, alanine, aspartate, and choline as key contributors to the variations and pathway analysis implicated alanine, aspartate and glutamate metabolism, and histidine metabolism linked to sexual maturity. Our study highlights metabolic changes during sexual maturation, identifying potential biomarkers for assessing reproductive maturity. These findings are initial steps toward optimising younger boars' usage in breeding, enhancing genetic gain, and reducing costs associated with their non-productive days at AI centres.

Intra-individual variability of the human seminal plasma metabolome

ABSTRACT

In contrast to the conventional spermiogram, metabolomics approaches give insights into the molecular composition of semen and may provide more detailed information on the fertility status of the respective donor. Given the intra-individual variability of spermiogram parameters between two donations, this study sought to elucidate the biological variability of the seminal plasma metabolome over an average period of 8 weeks. Two time-shifted semen samples from 15 healthy donors were compared by a targeted metabolomics approach utilizing the Biocrates AbsoluteIDQ p180 kit. Next to intraclass correlation coefficients (ICC), which represent a measure of reliability, coefficients of variation within individuals (CVW) and coefficients of variation between individuals (CVB) were calculated for each metabolite to demonstrate its stability. Furthermore, men were divided into two cohorts, a similar sperm concentration (SSC) and a differing sperm concentration (DSC) cohort, based on the observed variance in sperm concentration between the two semen donations. The ICC was higher in the SSC compared to the DSC cohort. The levels of 18 metabolites, primarily acylcarnitines, varied between the initial and subsequent donations. After subdivision into subgroups, only ornithine and phosphatidylcholine 40:5 exhibited differential levels between the two donations in the SSC group, compared to 14 metabolites in the DSC group. CVB was higher than CVW but both differed between the metabolite subclasses. Biogenic amines were identified as the least reliable analytes over time, exhibiting the highest CVW, compared to sphingomyelins, which demonstrated the highest reliability with the lowest variation. CVB was the highest for ether-bound glycerophosphatidylcholines and the lowest for amino acids.

The Association Between Lipid Serum and Semen Parameters: a Systematic Review

Increased lipid levels sometimes not only affect sexual function but also are considered to harm semen quality. It is often a suspicion that elevated lipids are a factor in infertility. We conduct a systematic review. Articles that met the criteria were identified according to The Preferred Reporting Items for Systematic Review and Meta-analysis of recommendations in the PubMed, ProQuest, EBSCO, Web of Science Wiley Online, Springer Link, Scopus, and Science Direct databases with no time restriction for publication. Seven studies are eligible for qualitative analysis from nine studies that have the potential to be assessed. These studies measure the correlation of serum lipids (VLDL, HDL, LDL, triglycerides, total cholesterol, free cholesterol, phospholipids, free fatty acids) with semen parameters (concentration, motility, morphology, DNA fragmentation index). Although not all studies consistently report that lipids impact semen quality, this review suspects that lipids have a significant impact on sperm quality. This study implies that it is necessary to maintain lipid levels to maintain sperm quality and quality of life. However, further investigation with an observational cohort study design needs to be carried out to assess the effect of lipids on semen quality more precisely for the promotion of reproductive health care.

A Metabolomic Profile of Seminal Fluid in Extremely Severe Oligozoopermia Suggesting an Epididymal Involvement

Male infertility has increased in the last decade. Pathophysiologic mechanisms behind extreme oligospermia (EO) are not yet fully understood. In new “omics” approaches, metabolomic can offer new information and help elucidate these mechanisms. We performed a metabolomics study of the seminal fluid (SF) in order to understand the mechanisms implicated in EO. We realized a targeted quantitative analysis using high performance liquid chromatography and mass spectrometry to compare the SF metabolomic profile of 19 men with EO with that of 22 men with a history of vasectomy (V) and 20 men with normal semen parameters (C). A total of 114 metabolites were identified. We obtained a multivariate OPLS-DA model discriminating the three groups. Signatures show significantly higher levels of amino acids and polyamines in C group. The sum of polyunsaturated fatty acids and free carnitine progressively decrease between the three groups (C > EO > V) and sphingomyelins are significantly lower in V group. Our signature characterizing EO includes metabolites already linked to infertility in previous studies. The similarities between the signatures of the EO and V groups are clear evidence of epididymal dysfunction in the case of testicular damage. This study shows the complexity of the metabolomic dysfunction occurring in the SF of EO men and underlines the importance of metabolomics in understanding male infertility.

Oxidative stress and phenotype frequencies of paraoxonase-1 in teratozoospermia

Oxidative stress causes infertility in men by affecting especially sperm morphology. The aim of the study was to examine the frequencies of paraoxonase-1 (PON1) phenotypes and the serum PON1, arylesterase, total antioxidant status (TAS), total oxidant status (TOS), oxidative stress index, catalase and thiol levels in teratozoospermic infertile men and normospermic fertile men. The study included 184 teratozoospermic infertile men and 72 normospermic fertile men. The double substrate method was employed to determine the phenotype distribution of PON1. The evaluation of sperm morphology was made in accordance with the Kruger's criteria. TAS, catalase and thiol levels were determined to be significantly lower in teratozoospermic infertile men compared to normospermic fertile men. A significant change was not observed in the levels of TOS, PON1 and arylesterase. There was a positive correlation between catalase and thiol levels and sperm morphology. While there were significantly more teratozoospermic men with AA phenotypes compared to normospermic men, there were significantly more persons with AB and BB phenotypes in normospermic men than in teratozoospermic men. As far as we know, such a study was conducted for the first time and suggests that PON1 phenotypic distribution may play a significant role in sterile males because of impaired sperm morphology.

Relationship between Lipids Levels of Serum and Seminal Plasma and Semen Parameters in 631 Chinese Subfertile Men

Objective

This prospective study was designed to investigate the relationship between lipids levels in both serum and seminal plasma and semen parameters.

Methods

631 subfertile men were enrolled. Their obesity-associated markers were measured, and semen parameters were analyzed. Also, seminal plasma and serum TC, TG, HDL and LDL and serum FFA, FSH, LH, total testosterone (TT), estradiol (E2) and SHBG levels were detected.

Results

Seminal plasma and serum TG, TC and LDL levels were positively related to age. Serum TC, TG and LDL were positively related to obesity-associated markers (P < 0.001), while only seminal plasma TG was positively related to them (P < 0.05). For lipids levels in serum and seminal plasma, only TG level had slightly positive correlation between them (r = 0.081, P = 0.042). There was no significant correlation between serum lipids levels and semen parameters. However, seminal plasma TG, TC, LDL and HDL levels were negatively related to one or several semen parameters, including semen volume (SV), sperm concentration (SC), total sperm count (TSC), sperm motility, progressive motility (PR) and total normal-progressively motile sperm counts (TNPMS). Moreover, seminal plasma TG, TC, LDL and HDL levels in patients with oligospermatism, asthenospermia and teratozoospermia were higher than those with normal sperm concentration, motility or morphology. After adjusting age and serum LH, FSH, TT, E2 and SHBG levels, linear regression analysis showed that SV was still significantly correlated with seminal plasma LDL (P = 0.012), both of SC and TSC with seminal plasma HDL (P = 0.028 and 0.002), and both of PR and sperm motility with seminal plasma TC (P = 0.012 and 0.051).

Conclusion

The abnormal metabolism of lipids in male reproductive system may contribute to male factor infertility.

A Pilot Comparative Study of 26 Biochemical Markers in Seminal Plasma and Serum in Infertile Men

Introduction. The relationships of the biochemical components in seminal plasma and serum, and their origins and physiological effects in male reproductive system have been poorly understood. Methods. Based on the calibration and quality control measures, 26 biochemical markers, in seminal plasma and serum samples from 36 male infertility patients with nonazoospermia were detected and compared. Results. Only PA was undetectable in all seminal plasma samples. There were significant differences of all other 24 biochemical markers in seminal plasma and serum (P < 0.05) except for UA (P = 0.214). There were rich proteins in seminal plasma, and globulin accounted for about 90%. There were also abundant enzymes in seminal plasma, and the activities of ALT, AST, AKP, GGT, LDH, CK, and αHBDH in seminal plasma were significantly higher than those in serum while ADA was inversely lower. There were relatively low levels of Glu, TG, TC, and hsCRP in seminal plasma, but Glu was undetectable in 8 of 36 cases. Conclusions. The differences of the levels of biochemical markers in seminal plasma and serum might be associated with the selective secretion of testis, epididymis and male accessory glands, and the specific environment needed for sperm metabolism and function maintenance.

Season-induced variation in lipid composition is associated with semen quality in Holstein bulls

Season-induced variation in fatty acid and cholesterol composition in bovine semen has been associated with semen quality. Given the specific roles of the various semen compartments (seminal fluids, sperm head, and sperm tail) in fertilization, we hypothesized that environmental-stress-induced alterations in the lipid composition of a specific compartment might impair semen quality and sperm function. Semen samples were collected from five mature Holstein–Friesian bulls during the summer (August to September) and winter (December to January). Semen was evaluated by computerized sperm-quality analyzer, calibrated for bulls' semen, and centrifuged to separate the spermatozoa from the seminal fluids. The spermatozoal fraction was sonicated to separate the sperm head and tail compartments. Cold lipid extraction was performed with chloroform:methanol (2:1, vol/vol). Lipids were identified and quantified by gas chromatography. Seasonal variation was found in both physiological and structural parameters. The proportion of spermatozoa defined as morphologically normal was higher in the winter, with higher motility, progressive motility, and velocity relative to summer samples. Lipid composition within fractions varied between seasons with prominent impairment in the tail compartment, characterized by high saturated fatty acid, low polyunsaturated fatty acid, and low cholesterol concentrations during the summer. Given the association between alterations in lipid composition and reduced sperm motility and velocity during the summer, it is suggested that lipid composition might serve to predict sperm quality.

Membrane fluidity and lipid content of human spermatozoa selected by swim-up method

In this work, we examined whether spermatozoa (spz) from normospermic fertile patients and selected by a swim-up (S-U) procedure had a particular membrane fluidity related to their maturity and their lipid content as compared with the sperm cells from the whole ejaculate (total sperm). Swim-up selected sperm had a reduced cytoplasmic space as revealed by a lower creatine kinase (CK) activity compared with total sperm (2 +/- 1 vs. 12 +/- 5 mUI/10(7) spz, p < 0.05). The cholesterol (Chol) and total phospholipid (PL) contents were significantly lower in S-U selected sperm than in total sperm (0.72 +/- 0.08 vs. 1.20 +/- 0.30 nmol/10(6) spz for Chol and 1.77 +/- 0.17 vs. 2.78 +/- 0.50 nmol/10(6) spz for PL, p < 0.05) and such a decrease was observed for the three major membrane PL: phosphatidylethanolamine (PE), phosphatidylcholine (PC) and sphingomyelin (SM). However, these decreases were not associated with a change in either Chol/PL or PC/(PC + PE) molar ratios. Membrane fluidity estimated by fluorescence polarization remained comparable between the S-U sperm fraction and total sperm (fluorescence polarization anisotropy, r, which is inversely proportional to the fluidity: 0.235 +/- 0.006 vs. 0.230 +/- 0.005). The sperm membrane fluidity obtained in normospermic patients was compared with abnormospermic ones (oligoasthenoteratospermia). In abnormospermic patients, the membrane fluidity was decreased in migrated spermatozoa compared with total sperm (anisotropy: 0.210 +/- 0.010 vs. 0.250 +/- 0.013, p < 0.01). Our data suggest that the S-U method selected a subpopulation of mature spermatozoa characterised by a low content of Chol and PL, likely related to a reduced membrane area. The fact that Chol/PL and PC/(PC + PE) molar ratios were unchanged shows a maintenance of the membrane quality. This was confirmed by the fluorescence anisotropy measurement showing no difference in plasma membrane fluidity between S-U selected sperm and total sperm. In abnormal semen the migrated spermatozoa had a lower fluidity compared with total sperm suggesting a defective sperm function. These results bring new elements characterizing the S-U selected spermatozoa.

Laurdan fluorescence: a simple method to evaluate sperm plasma membrane alterations

OBJECTIVE

To determine, by a simple fluorescence method, sperm plasma membrane alterations related with changes of lipid bilayer that, together with routine semen analysis, could help to elucidate the causes of the unexplained male infertility problems.

DESIGN

Pilot study.

SETTING

Andrology laboratory and biochemistry institute, medical school.

PATIENT(S)

Men whose semen was studied for infertility problems.

INTERVENTIONS(S)

No therapeutic intervention was performed on patients.

MAIN OUTCOME MEASURE(S)

Presence of spermatozoa plasma membrane alterations evidenced by evaluation of Laurdan fluorescence Generalized Polarization (GP) and reported as a function of increasing cell concentration, spermatozoa total motility, linear speed, and vitality.

RESULT(S)

Reporting GP values as a function of increasing sperm cell concentration, it is evident that the samples are distributed in two distinct areas: at >32 x 10(6) cells per milliliter, mean GP value was 0.303 +/- 0.015, whereas for lower sperm cell concentrations, the mean GP was 0.365 +/- 0.026 (P<.001). These data indicate that the spermatozoa plasma membranes are characterized by liquid-crystalline phases with different ordering degree and polarity and that about 50% of samples with normal semen characteristics (> or =20 x 10(6) cells per milliliter) show high GP values.

CONCLUSION(S)

Laurdan fluorescence can be used as a simple method to evaluate spermatozoa plasma membrane alterations, particularly in a group of infertile men presenting normal semen parameters. In these samples, Laurdan could be used as a simple tool for infertility assessment. In fact, it is known that compositional and physicochemical alterations of bilayer features can be important for the fertilizing ability of spermatozoa because they are necessary for a proper physiological membrane activity.

Electron paramagnetic resonance spectroscopy for the investigation of the fluidity of human spermatozoa plasma membranes: a feasibility study

Spermatozoal membrane perturbations may play a role in abnormal sperm functions. The objective of this investigation was to study the feasibility of measuring membrane fluidity of isolated human sperm by electron paramagnetic resonance (EPR) spectroscopy and to compare the order parameter of spectra obtained from the sperm plasma membranes of living sperm of fertile men with that of infertile men. Ejaculates of infertile and fertile men were washed and the spermatozoa labelled with 5-doxylstearic acid (5-DSA) and 16-doxylstearic-acid (16-DSA) (10 nmol per 4 x 10(7) sperm). The reporter group of 5-DSA partitions into the outer, hydrophilic part of the sperm plasma membrane, whereas that of 16-DSA is distributed in the inner hydrophobic part. The following results were obtained: (i) the lowest measurable cell count was 3.6 to 7 x 10(6) sperm and the interassay variance of the order-parameter s was < 1%; (ii) swim-up experiments revealed a higher fluidity of sperm with a higher percentage of motility; (iii) sperm membranes of infertile patients exhibited a decreased fluidity of their plasma membranes in the polar interface region of 5-DSA compared with volunteer semen donors and fertile men (P=0.002). No difference of membrane fluidity was found between the different groups using 16-DSA. It is concluded that EPR spectroscopy can be used to study the fluidity of sperm plasma membranes in fertile and infertile men.

Separation and quantification of cholesterol and major phospholipid classes in human semen by high-performance liquid chromatography and light-scattering detection

A high-performance liquid chromatographic method coupled with light-scattering detection for the separate and accurate quantification of cholesterol and main phospholipid classes was applied to human spermatozoa and seminal plasma (SP). This method is based on normal-phase chromatography with silica gel as stationary phase and a ternary gradient with hexane, mixtures of chloroform-methanol and water as mobile phase. Lipids are separated with a good resolution and a high reproducibility. About 5 x 10(6) spermatozoa or 25 microl of seminal plasma are sufficient to accurate quantitative analysis of phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidycholine (PC), sphingomyelin (SM) and cholesterol. PC is the predominant phospholipid class in spermatozoa (102+/-8 nmol/10(8) spermatozoa) whereas SM is the major in the SP (163+/-6 nmol/ml). Both in spermatozoa and SP, PI is the minor class of the phospholipids (12+/-1 nmol/10(8) spermatozoa and 24+/-2 nmol/ml). In conclusion, this method offers interesting perspectives for analysis of sperm lipid composition in semen samples with low quantities of spermatozoa.

Correlation between sperm membrane destabilization by heparin and aniline blue staining as membrane integrity index

Acidic aniline blue stain (AAB) was studied in relation to sperm membrane destabilization and nuclei decondensation by heparin. Untreated spermatozoa smears stained with AAB or vital stain shows 28.4% of stained and 71.6% of unstained nuclei. This behavior was also observed when incubation was done in the presence of 5 mM glutathione (GSH) used alone. In the presence of 21.6 microM heparin, staining of sperm cells commenced 10 min after heparin addition and was dependent on the incubation time. During the experiment 12.3% of the total cholesterol content and 20 micrograms protein/10(8) sperm cells were released. In the presence of 21.6 microM heparin-5 mM/GSH, swelling of sperm nuclei reach 95% after 150 min incubation. When this experiment was run along with AAB, the same average (45%) was seen in the first 30 min, which gives plenty of time to trigger the nuclei's decondensation mechanism. The percentage of stained cells was of 71%, indicating that the histone is not completely replaced, and insuring a positive reaction with AAB stain. It would appear that AAB stain can be used as a membrane integrity index to confirm the destabilization effect of heparin on the sperm membrane.

Cholesterol, phospholipids and markers of the function of the accessory sex glands in the semen of men with hypercholesterolaemia

The effect of hypercholesterolaemia on the cholesterol and phospholipid content of spermatozoa and seminal plasma was studied. Testosterone and specific markers of the accessory sex glands were also measured. Semen samples from 11 hypercholesterolaemic patients (plasma cholesterol > 6.42 mmol/l, plasma triglycerides < 2 mmol/l) were compared with those of 11 normocholesterolaemic controls (plasma cholesterol < 5.14 mmol/l, plasma triglycerides < 2 mmol/l). Cholesterol, phospholipids and the molar ratio of cholesterol: phospholipids were not significantly different between the two groups of patients either in spermatozoa or in seminal plasma. In hypercholesterolaemic patients the total amount of carnitine in the ejaculate was significantly higher, but there were no significant differences in the levels of acid phosphatase or fructose. There were no significant differences in seminal plasma levels of testosterone in the two groups of subjects. These results suggest that hypercholesterolaemia has no effect on cholesterol and phospholipid levels in spermatozoa and does not cause gross modification of the secretory function of the accessory sex glands.

Effect of chronic alcoholism on semen--studies on lipid profiles

The effect of chronic alcoholism on various seminal parameters (sperm concentration, rate of forward motility, percentage of abnormal spermatozoa, lipid profiles of seminal plasma and spermatozoa) was studied together with the serum levels of testosterone and oestradiol. In chronic alcoholics there was a marked reduction in sperm concentration and in the rate of their forward motility, and increase in the number of spermatozoa with morphological abnormalities when compared to age-matched normal fertile subjects. Serum levels of testosterone were decreased while oestradiol levels were increased in chronic alcoholic men. Studies of lipid profiles showed a marked decrease in the total phospholipid concentration in spermatozoa, primarily in sphingomyelin, phosphatidyl choline and ethanolamine fractions. The cholesterol:phospholipid ratio in spermatozoa was increased in alcoholics. In the seminal plasma of chronic alcoholics, there was a decrease in total lipid, in glyceride glycerol and in free and esterified cholesterol. Of the phospholipid classes, sphingomyelin and phosphatidyl ethanolamine showed a significant reduction. In general, the present study provides evidence for the adverse effects of chronic alcoholism on serum hormones, sperm count, morphology, motility and seminal lipid profiles. These may be responsible for the fertility disorders common in chronic alcoholics.

Cholesterol and polyunsaturated acid enriched diet: effect on kinetics of the acrosome reaction in rabbit spermatozoa

In this study we examined the effect of cholesterol (Diet 2), cholesterol and fish oil (FO) polyunsaturated acid (Diet 3), and polyunsaturated acid (Diet 4) enriched diets upon the acrosome reaction (AR) of New Zealand White rabbit spermatozoa. Male rabbits fed with cholesterol alone or with FO increased their cholesterol and LDL-cholesterol serum levels after 15 days of diet. Ten semen samples were obtained after 2 months of diet. Our results suggest that hypercholesterolemia and hypertriglyceridemia in male rabbits could produce a decreased capacity of sperm AR after 4 h (0%, 0%, and 60% lower than the control), 6 h (0%, 68%, and 44%), or 8 h (58%, 52% and 32%) of incubation in capacitating medium. Another set of experiments were made with lysophosphatidylcholine (LPC), 80 micrograms/ml, and the same pattern of AR was seen. Nevertheless, the high cholesterol and total lipids (TL) levels in serum did not affect the cholesterol levels in seminal plasma (SP) but affect the SP total lipids. The diminished capacity of rabbit sperm to undergo the AR was not reverted by in vitro incubation with the Shinitsky medium for cholesterol depletion (MDC). These results indirectly suggest that the cholesterol/phospholipid ratio in hypercholesterolemic sperm is similar to that of controls and are in agreement with preliminary studies made in our laboratory that evidenced the same cholesterol/phospholipid ratio in rabbit sperm from hypercholesterolemic animals than from controls.(ABSTRACT TRUNCATED AT 250 WORDS)

On the glandular origin of seminal plasma lipids in man

The aim of this study is to examine the glandular origin of seminal fluid lipids in man. The triglycerides, the total cholesterol, the non-esterified fatty acid and the total phospholipids were measured in seminal plasma of vasectomized patients (n = 8) and control subjects (n = 15). The same parameters were measured in seminal plasma collected in three fractions from split ejaculates (n = 10). The total cholesterol and the non-esterified fatty acid are principally prostatic in origin. The phospholipids are secreted by the epididymis but also by the prostate. The origin of the triglycerides seems to be very varied.

Differences in lipoprotein composition between heads and tails of human sperm: an infrared spectroscopy study

Human spermatozoa and their fractions (heads and tails) have been studied by infrared spectroscopy. Protein conformation in isolated human spermatozoa heads, although predominantly of the alpha helix or random coil type, has a significant proportion of antiparallel B structure. Spectra of isolated spermatozoa tails show that proteins exist in this fraction preponderantly in pleated-sheet conformation (parallel and antiparallel). The quantity and type of lipids seem to be drastically different between heads and tails of spermatozoa. Head lipids are scarce and difficult to extract, and they are apparently tightly bound to proteins, highly unsaturated, and rich in free hydroxyl and carboxyl groups. Tail lipids are more abundant and more easily extractable. Head phospholipids are probably phosphatidylcholine, cephalins, and inositols, and tail phospholipids are preponderantly plasmalogen-type lecithins and sphingomyelins. The presence of specific infrared bands points to the existence in tails of important amounts of sulfur compounds, probably sulfolipids or sulfoglycolipids.

Identification of sterol acceptors that stimulate cholesterol efflux from human spermatozoa during in vitro capacitation

The nature of cholesterol-binding proteins acting upon human spermatozoa during in vitro capacitation was determined by measuring the efflux of [3H]cholesterol and of [3H]cholesteryl sulfate from labeled spermatozoa. Efflux of [3H]sterols was stimulated when the labeled gametes were incubated in Ham's F-10 medium supplemented with female serum or follicular fluid. Upon centrifugation of capacitated spermatozoa and application of the supernatant to density-gradient ultracentrifugation for lipoprotein analysis, both [3H]cholesterol and [3H]cholesteryl sulfate were found to be carried by very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), high-density lipoproteins (HDL), as well as the albumin fraction (d greater than 1.21) in serum. When the capacitation medium was supplemented with follicular fluid, the [3H]sterols were bound to HDL's and to the albumin fraction; when the latter fraction was analysed by molecular sieve chromatography, 60-70% of the radioactivity eluted in fractions with a mean molecular weight corresponding to that of human serum albumin. Sperm cholesterol efflux was also stimulated when serum or follicular fluid was added to a simplified medium (50 mM Tris-HCl, 0.56% NaCl, pH 7.8); efflux of [3H]cholesterol from labeled gametes progressed in a time-dependent manner, but was low in the absence of serum components. The [3H]cholesterol/cholesterol ratios were higher in the albumin and HDL fractions, indicating some degree of specificity of these sterol acceptors. It was observed that follicular fluid albumin has a [3H]sterol binding capacity that is 2-3-fold higher than that of serum albumin. Commercial human serum albumin also promoted sperm cholesterol efflux. These results provide new information concerning those components of follicular fluid which may play a role in human sperm capacitation and provide further support for the concept that loss of cholesterol from the sperm plasma membrane is an important component of the capacitation process.