Surface protein changes in goat spermatozoa during capacitation

Polypeptides of goat sperm surface before and after capacitation were examined by radiolabelling and immunologically using polyclonal antisera. Radioiodination revealed five protein bands having mol wt of 14.8, 72.4, 81, 100 and 128 kDa in uncapacitated ejaculated spermatozoa and only three bands of 23.4, 27 and 72.4 KDa in capacitated spermatozoa. The protein band with mol wt 72.4 kDa was only feebly iodinated in uncapacitated sperm surface but in capacitated spermatozoa it was heavily labelled. Western blot analysis of detergent-extracted proteins using gamma-globulin fraction of antisera raised against purified goat sperm plasma membrane revealed six antigens (17.8, 29.1, 33.4, 45.6, 85.1, 123.2 kDa) in uncapacitated spermatozoa, four (26, 32.1, 40.1, 45.6 kDa) in capacitated spermatozoa and only one (45.6 kDa) in acrosome-reacted spermatozoa. High mol wt proteins were more numerous on the surface of uncapacitated spermatozoa while the capacitated spermatozoa had relatively low mol wt proteins. An apparent effect of capacitation is the metabolism and reorganisation of proteins on goat sperm surface. Polypeptides on capacitated sperm surface revealed through radiolabelling and polyclonal antisera may have a likely receptor(s) role in the recognition and binding to homologous zona pellucida during fertilization.     

In vitro capacitation of bull spermatozoa by oviductal fluid and its components

Sperm capacitation is crucial for fertilization. However, debate continues on exactly how, where and when capacitation is elicited in the bovine female genital tract. In this study we used merocyanine-540 and the chlortetracycline (CTC) assay to test how capacitation of bull spermatozoa is affected in vitro by exposure to oviductal fluid (ODF) collected in vivo, various glycosaminoglycans (GAGs) or bicarbonate. Following different durations of exposure, spermatozoa were stained with CTC or merocyanine-540, and evaluated with epifluorescent light microscopy or flow cytometry, respectively. Incubation time did not significantly affect capacitation. Exposure (30-120 min) to ODF capacitated (p < 0.05) bull spermatozoa as measured by either merocyanine-540 or CTC. Hyaluronan was the only GAG that induced a significant increase in B-pattern spermatozoa (capacitated; p = 0.012) compared with controls. Dermatan sulphate also induced capacitation (merocyanine-540 high fluorescence; p = 0.035). Exposure to bicarbonate-enriched media also yielded an increase in merocyanine-540 high fluorescence (p < 0.0001). When bicarbonate was added to the other treatments (ODF or GAGs) an equal increase in merocyanine-540 high fluorescence was noted (p < 0.0001), compared with before addition of bicarbonate and independent of the treatment before exposure. There was no significant difference in the number of B-pattern spermatozoa when bicarbonate was added, but an significant increase in spermatozoa with an acrosome-reacted (AR)-pattern (p < 0.0001) was observed. Exposure of spermatozoa to solubilized zonae pellucidae significantly increased the AR-pattern spermatozoa (p = 0.016). In conclusion, ODF was more potent in inducing capacitation of bull spermatozoa than the individual GAGs. Our results also indicate that bicarbonate is an effector of bull sperm capacitation.     

Microfilaments appear in boar spermatozoa during capacitation in vitro

Boar spermatozoa were incubated in a capacitation medium and examined for the presence of filamentous actin by using the fluorescent probe NBD-phallacidin. F-actin was not observed in uncapacitated sperm, but developed in most regions of the cell during the capacitation period. Fluorescent staining was most intense in the flagellum. When fresh seminal plasma was added to capacitated sperm and the sperm was further incubated, F-actin was no longer observed. In view of previous experiments which indicated that plasma membrane proteins (PMPs), including a major integral PMP, move out of the sperm head into the flagellum during capacitation and that this movement is inhibited by the microfilament poison cytochalasin D (Peterson, Saxena, Saxena, and Russell: Biol. Reprod., in press, '86), we suggest that actin-PMP interactions play a major role in capacitating boar spermatozoa.     

Individual differences in capacitation of bull spermatozoa by heparin in vitro: relationship to fertility

Several parameters of motility and integrity of frozenthawed spermatozoa are compared with the ability of selected motile, intact spermatozoa to acrosome reaction induced by 0.005% hyamin. Between semen donors there exist distinct individual differences; however only the induced acrosome reaction after heparin treatment showed a significant correlation with fertility. For 12 bulls the nonreturn rates from 334 to 559 services correlated significantly with induced acrosome reaction (r = 0.607). The in vitro fertilization of 53 to 93 tubal bovine oocytes with frozen-thawed spermatozoa from five bulls yielded a correlation of r = 0.621 with the rate of induced acrosome reaction. The different capacity levels of heparin-treated spermatozoa to undergo acrosome reaction appears to correspond to the varying intensity and kinetics of heparinmediated head-to-head aggregation of motile cells. The applied functional parameters could be used for the selection of bulls with low fertility in artificial insemination programs, and for spermatozoa donors for in vitro fertilization.     

Cyclic nucleotide metabolism in mouse epididymal spermatozoa during capacitation in vitro

The role of cyclic nucleotides in sperm capacitation is equivocal. Using conditions known to support mouse sperm capacitation after 120 min incubation in vitro, the cAMP and cGMP contents of epididymal spermatozoa were measured and the cGMP/cAMP ratio determined. The initial high cAMP content detected upon release of spermatozoa decreased within 30 min to a lower plateau, which was then maintained throughout incubation. With the cGMP content remaining approximately constant, the cGMP/cAMP ratio increased over 120 min. In the presence of 2 mM caffeine, an increased cAMP content was noted at 0 and 30 min before a fall to the plateau level. To investigate cyclic nucleotide metabolism, adenylate cyclase and phosphodiesterase activities were compared in two sperm populations, one essentially uncapacitated and the other incubated for 120 min. Adenylate cyclase activity, higher in the presence of 2 mM Mn²⁺ compared to Mg²⁺, showed increased activity at 120 min compared to 30 min incubation, while phosphodiesterase activity decreased during this period. The ability of spermatozoa to form adenosine and inosine from cAMP indicated endogenous 5′-nucleotidase and deaminase, as well as phosphodiesterase, activities. Although the endogenous cAMP content appeared to remain constant during the time that acrosome loss, hyperactivated motility and fertilizing ability can be demonstrated, activities of the enzymes responsible for cAMP metabolism indicate an increased potential for cAMP availability and turnover. The increased cGMP/cAMP ratio may also play a role during capacitation.     

In vitro capacitation of goat spermatozoa

Goat epididymal and ejaculated spermatozoa were incubated in Krebs-Ringer bicarbonate buffer containing pyruvate and lactate as energy source. A 3 hr incubation for epididymal and 4 hr for ejaculated spermatozoa was required for the capacitation and acrosome reaction to take place. Calcium is an essential requirement which was needed for motility maintenance/activation and for the initiation of acrosome reaction. A 2-fold increase in cAMP content was measured over 3 hr period of incubation of goat epididymal spermatozoa which was not seen when calcium ions were either omitted or chelated with EGTA. There is thus a definite involvement of Ca2+ ions and cAMP in capacitation and acrosome reaction of goat spermatozoa.     

Evaluation of in vitro capacitation of stallion spermatozoa

The primary aim of this study was to establish a flow cytometric technique for determining the capacitation status of stallion spermatozoa. To this end, a flow cytometric technique that demonstrates changes in plasma membrane fluidity; namely, merocyanine 540 staining, was compared with the more conventional Ca(2+)-dependent fluorescence microscopic technique, chlortetracycline (CTC) staining, for assessing capacitation status. In addition, the effect of bicarbonate/CO(2) on the progress of capacitation and the acrosome reaction (AR) and on temporal changes in sperm motility, with particular regard to hyperactivation, was analyzed. For the study, fresh semen was washed and then incubated for 5 h in bicarbonate-containing or bicarbonate-free medium, with or without Ca(2+) ionophore to induce the AR, and at intervals during incubation aliquots were taken and analyzed for capacitation and acrosome status. The AR was assessed using both the CTC and fluorescein isothiocyanate-peanut agglutinin (FITC-PNA) staining techniques with similar results. In brief, it was found that merocyanine 540 detects capacitation-related changes much earlier than CTC does (0.5 h versus approximately 3 h), and that flow cytometry for evaluation of capacitation and AR was a quicker (10 sec per sample) and more accurate (10,000 cells counted) technique than fluorescence microscopy. Furthermore, it was observed that Ca(2+) ionophore could not induce the AR in the absence of bicarbonate, but that the ionophore synergized the bicarbonate-mediated induction of the AR as detected by CTC (although it was not significant when evaluated using FITC-PNA). The percentage of hyperactive sperm in each sample was not affected by time of incubation under the experimental conditions studied. In conclusion, merocyanine 540 staining is a better method than CTC staining for evaluating the early events of capacitation for stallion spermatozoa incubated in vitro. Furthermore, bicarbonate sperm activation clearly plays a vital role in the induction of the AR in stallion spermatozoa.     

In vitro capacitation of Siberian tiger spermatozoa

Three experiments were designed to determine optimum conditions for capacitation of Siberian tiger (Panthera tigris altaica) sperm in vitro using the zona-free hamster egg sperm penetration assay (SPA) as a verification of capacitation. Sperm collected from a 9-year-old captive Siberian tiger were subjected to different in vitro washing conditions, preincubation times, and temperatures to induce capacitation. Sperm were able to penetrate zona-free hamster ova after 2 hours preincubation at 37°C but not at time 0. Preincubation at room temperature was not sufficient to prepare sperm for fertilization. The presence of seminal plasma during the 2-hour, 37°C preincubation did not affect the ability of tiger sperm to penetrate zona-free hamster eggs. The SPA can provide a means for evaluation of in vitro capacitation of Siberian tiger sperm.     

Energy requirement of bovine spermatozoa for in vitro capacitation

The oxidative energy requirements of bovine spermatozoa capacitated with dilauroil-phosphatidylcholine liposomes (PC 12) and the effect of these liposomes on acrosome reaction necessary for in vitro fertilization were studied. Mitochondrial respiration was measured using 3 different substrates (pyruvate-lactate-glucose) and endogenous substrates. The samples were either treated with PC 12 or were left untreated and used as the control. A 2.8-fold increase in the consumption of oxygen was observed in the PC 12 treated spermatozoa in the presence of the 3 combined substrates (pyruvate-lactate-glucose). Respiration changes were not observed when the spermatozoa were capacitated with only 2 of the 3 substrates or with glucose alone. When endogenous substrates were used, the consumption of oxygen increased 1.7 times, and mitochondrial uncoupling was observed in the treated samples. The hypermotility characteristic of the capacitation process was not observed when glucose or endogenous substrates were used. When the percentage of intact acrosomes was determined using differential-interferential contrast (DIC) microscopy, it was found that in the presence of oxidative substrates there was a 26% decrease compared with that of the control sample. The proportion of reacted acrosomes was in the range of 41.3 to 49.6%, as measured by the chlortetracycline epifluorescence method in the presence of calcium ionophore A23187. Only 4% of the spermatozoa showed acrosome reaction with endogenous substrates. A higher percentage of fertilized oocytes were observed when the spermatozoa were capacitated in the presence of the 3 substrates (pyruvate-lactate-glucose), confirming that the success of in vitro fertilization depends on the energy conditions associated with the capacitation process. The results of these experiments indicate that the presence of oxidative energy is necessary to produce capacitation and the hyperactivation characteristic in frozen-thawed bovine spermatozoa treated with liposomes.     

Role of nitric oxide on quality of freshly ejaculated bull spermatozoa during heparin-induced in vitro capacitation

The objective of this study was to assess the effects of nitric oxide (NO) on heparin-induced capacitation in vitro of fresh bull sperm, through the addition of Nω-nitro-l-arginine methyl ester (L-NAME, a NO-synthesis inhibitor) and l-arginine (L-Arg, a NO-synthesis precursor) to the capacitation medium. In Experiment 1, different concentrations of L-NAME (0.1, 1, 10 mM) and of L-Arg (10 mM) were added to the capacitation medium. Sperm motility and vigor were subjectively appraised using direct light microscopy; sperm membrane integrity was examined using a 2% Trypan blue solution while the concentration of nitrate/nitrite (NO₃⁻/NO₂⁻) was determined by using the Griess method over a 5 h capacitation period. The addition of 10 mM L-NAME has inhibited NO synthesis, sperm progressive motility, sperm vigor and sperm membrane integrity (P < 0.05) as compared to control. The addition of 10 mM L-Arg to the capacitation medium increased all variables evaluated in comparison to the control (P < 0.05). In Experiment 2, mitochondrial activity was assessed through the MTT test (3-(4,5-dimetylthiazol-2-yl)-2,5-diphenyltetrazolium bromide), and sperm capacitation was assessed through the test of penetration in homologous oocytes after addition of the 10 mM L-NAME, and of the 10 mM L-Arg. The addition of 10 mM L-NAME caused mitochondrial activity (40%) and the percentage of oocytes penetrated (77%) to decrease in relation to the control (P < 0.05). After addition of 0.6 mM L-Arg + 10 mM L-NAME, partial reversal of mitochondrial activity did occur (only 20%). The addition of 10 mM L-Arg increased the percentage of oocytes penetrated as compared to control (21%) (P < 0.05). These results indicate that: (1) NO is involved in control of progressive sperm motility, vigor, membrane integrity, and mitochondrial activity along the period of heparin-induced capacitation of fresh bovine sperm via NOS/NO; (2) adequate L-Arg/NO concentrations into the capacitation medium can potentiate heparin action or act independently for increasing the number or the quality of capacitated sperm.     

Topographical rearrangement of a plasma membrane antigen during capacitation of rat spermatozoa in vitro

We have previously described an antigen (termed 2B1) on rat spermatozoa that is present on the plasma membrane overlying the tail domain. The antigen is mobile within the plane of the plasma membrane and a mAb to it blocks fertilization in vitro. In the present study we describe some dynamic properties of this antigen in relation to its topographical distribution. When spermatozoa were incubated in vitro in a capacitation medium and stained with 2B1 mAb/FITC-rabbit anti-mouse F(ab')2, strong fluorescence appeared over the acrosomal domain. Acute exposure of fresh spermatozoa to dissociating reagents (1 M NaCl or 5 mM 2-mercaptoethanol) or inducers of the acrosome reaction (lysolecithin + Ca2+ or A23187 + Ca2+) failed to mimic these effects. Spermatozoa prelabeled with FITC-2B1 IgG and then capacitated in the presence of excess "cold" 2B1 IgG also showed accumulation of fluorescence on the acrosomal domain, suggesting that the antigen had migrated from the tail. Migration was selective and Ca2(+)- and temperature-dependent but was not inhibited by metabolic poisons (NaF or NaN3). Motility was not obligatory for migration. Immunogold-labeling studies at the ultrastructural level showed that 2B1 antigen was restricted to the surface membrane over both the tail and the acrosomal domains and that during migration it did not change the type of membrane into which it was inserted. From a quantitative analysis of fluorescence on spermatozoa prelabeled with FITC-2B1 IgG and then capacitated, the amount of antigen that appeared on the acrosomal domain was approximately equivalent to that lost from the midpiece domain. The Mr of 2B1 antigen extracted from capacitated spermatozoa was 300-500 Da less than that extracted from noncapacitated cells, suggesting that the molecule had undergone processing concomitant with migration. These results are discussed in relation to mechanisms for targeting antigens to sites where they become physiologically active and are correctly positioned to participate in gamete recognition processes.     

Phospholipase A2 activation by hydrogen peroxide during in vitro capacitation of buffalo spermatozoa

Progressively motile, washed buffalo spermatozoa (50 x 10(6) cells in 0.5 ml) were in vitro capacitated in HEPES containing Bovine Gamete Medium 3 (BGM3) in presence of heparin (10 microg/ml), and different concentrations of hydrogen peroxide (10 to 100 microM). Spermatozoa (60%) were capacitated in presence of heparin compared to 56% in presence of 25 microM H2O2 (optimally found suitable for capacitation). The extent of capacitation was measured in terms of acrosome reaction (AR) induced by lysophosphatidyl choline (100 microg/ml). The acrosome reacted cells were counted after triple staining. Catalase (100 microg/ml) significantly reduced the sperm capacitation to 16-18% when added with H2O2, or alone in the capacitation medium. Phospholipase A2 activity of spermatozoa increased linearly up to 50 microM H2O2 concentration included in the assay system. Moreover, significant increase in phospholipase A2 activity was observed after capacitation by both, the heparin and 25 microM H2O2. The activity was always higher in acrosome reacted cells.     

Surface alterations during the capacitation of mammalian spermatozoa

Capacitation is the process by which mammalian sperm acquire the ability to undergo the acrosome reaction which, in turn, is a prerequisite for sperm-egg fusion and penetration. Until recently, it was thought that capacitation involved subtle physiological and chemical changes which had no morphological counterparts even at the electron microscopic level. However, it has now been shown by a number of investigators that material associated with the plasma membrane surface is either lost or extensively redistributed during in vitro or in vivo capacitation. We have made use of lectins and antibodies as probes of the sperm surface during capacitation and the acrosome reaction. Concanavalin A (Con A), wheat germ agglutinin (WGA) and soybean agglutinin (SBA) have been used in conjunction with fluorescent tags (FITC) and ultrastructural markers (ferritin, hemocyanin) to study the surface of golden hamster, guinea pig, mouse and human spermatozoa. Con A and WGA label the plasma membrane overlying the acrosomal region quite uniformly on these species. After capacitation there is a specific loss (or masking) of lectin binding sites over the acrosomal region of the sperm head in all species examined. Antibodies prepared against sperm and specific antibodies to a cell surface protein (fibronectin) were also tagged with fluorescent or ultrastructural markers and used to label the surfaces of sperm before and after capacitation. These probes also indicate a specific loss of surface associated material over the acrosomal surface after capacitation. These results are consistent with the notion that there is a general removal of surface components during capacitation and that this denuding of the surface is a prerequisite for the following membrane fusion events involved in the acrosome reaction and sperm-egg fusion.     

Heparin and dermatan sulphate induced capacitation of frozen-thawed bull spermatozoa measured by merocyanine-540

Glycosaminoglycans (GAGs) are present in the oviduct in which the major part of sperm capacitation occurs. In this study we have tested how capacitation of frozen-thawed bull spermatozoa is effected by exposure to different GAGs detectable or possibly present in oviductal fluid; i.e. heparin, hyaluronan, heparan sulphate, dermatan sulphate and chondroitin sulphate. Following exposure of different duration, the spermatozoa were stained with either Chlortetracycline (CTC) or merocyanine-540 and evaluated with epifluorescent light microscopy or flow cytometry, respectively. Heparin elicited a significant increase in the number of alive, capacitated spermatozoa, either expressed as higher merocyanine-540 fluorescence (p < 0.0001) or as B-pattern (p = 0.0021) in the CTC assay, during 4 h of incubation. When comparing the different GAG treatments one by one to the negative control in the flow cytometric study, only heparin and dermatan sulphate were significant (p < 0.0001) higher than the control at 0-30 min of incubation. Duration of incubation did not affect the proportion of capacitated spermatozoa when measured as merocyanine-540 fluorescence or CTC B-pattern, but the length of the incubation did affect the number of dead (Yo-PRO 1 positive) spermatozoa (p < 0.0001). Exposure to zona pellucida proteins significantly increased the proportion of acrosome reacted spermatozoa (p = 0.016). Both heparin and dermatan sulphate induce capacitation of frozen-thawed bull spermatozoa in vitro.     

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 (VLDL), high-density lipoproteins (HDL), as well as the albumin fraction (d > 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.     

Dibutyryl cyclic AMP decreases capacitation time in vitro in mouse spermatozoa

Epididymal mouse sperm suspensions were preincubated for various times in medium containing glucose and/or dibutyryl cyclic AMP and then assessed for fertilizing ability in vitro, loss of the acrosome and motility changes. Capacitation time was significantly reduced by exposure to glucose and 0.1 mM-dbcAMP for 30 min as evidenced by early and synchronous fertilization of eggs, compared with glucose alone. Although this was accompanied by a precocious development of whiplash motility, the rate of acrosome loss in isolated sperm suspensions was not accelerated by the presence of exogenous cyclic nucleotide. Exposure of spermatozoa to 1 mM-dbcAMP in the presence of glucose resulted in very poor fertilization, but the effect could be prevented by withholding glucose until eggs were introduced; this may be due to free butyrate in the system since the inclusion of 1 mM-butyrate in glucose-containing medium had a similar inhibitory effect. Although cyclic nucleotide supported the acrosome reaction but not motility changes, no fertilization was obtained unless zonae were removed, when a low level of fertilization (30%) was observed. Both whiplash motility and acrosome loss are thus obligatory for sperm penetration of the zona and glycolytic metabolism supports both changes, perhaps by promoting endogenous generation of cyclic AMP to act as an intermediary in these two distinct phenomena.     

Importance of glycolysable substrates for in vitro capacitation of human spermatozoa

To investigate the importance of glycolysable substrate for supporting the ability of human sperm to capacitate and penetrate oocytes in vitro, washed spermatozoa were incubated with or without various sugars in BWW culture medium containing pyruvate and lactate. Sperm penetration was assayed using zona-free hamster oocytes. After an 18-h preincubation, glucose (1 mg/ml) supported higher penetration of sperm into oocytes than either mannose or fructose (60.7% vs. 28.2% or 21.5%, respectively) at the same concentration. Penetration was even lower when medium contained the nonmetabolizable sugar galactose (2.1% at 1 mg/ml). On the other hand, higher concentrations (5 or 10 mg/ml) of glucose, but not fructose, suppressed penetration, provided the glucose was present throughout the 18-h preincubation. When caffeine, a stimulant of glycolysis in human sperm, was present along with glucose, sperm penetration was enhanced, but only after 6 h of sperm preincubation. This effect was not observed in glucose-free medium, however, where penetration remained low over a 10-h incubation period. In these experiments, the percentage of motile sperm was unaffected by treatment, but the quality of motility was diminished in the absence of glucose. We conclude that stimulation of glycolysis may promote capacitation of human spermatozoa in vitro and that optimization of penetrating ability of sperm is dependent upon both the type and concentration of glycolysable sugar present.     

Acrosomal enzymes of human spermatozoa before and after in vitro capacitation

The effect of in vitro capacitation (events that occur before the acrosome reaction) on the acrosomal enzymes of human spermatozoa was determined. Capacitation of human spermatozoa was assessed by their ability to penetrate denuded hamster oocytes. The activities of a number of enzymes commonly associated with the sperm acrosome, including nonzymogen acrosin, proacrosin, inhibitor-bound acrosin, hyaluronidase, acid phosphatase, beta-glucuronidase, beta-glucosidase, beta-N-acetylglucosaminidase, beta-galactosidase and beta-N-acetylgalactosaminidase were assessed. With the exception of acid phosphatase, no alteration in enzyme activity occurred after 4 h of incubating the spermatozoa under capacitation conditions although gamete fusion took place. The acid phosphatase levels decreased twofold, presumably due to the loss of seminal (prostatic acid phosphatase that loosely adheres to spermatozoa. After 8 h of capacitation, a large decrease in sperm enzyme levels took place only in the case of hyaluronidase, although small decreases were also noted in total acrosin, proacrosin and inhibited acrosin. No new electrophoretically migrating forms of acrosin were observed. Decreases in total acrosin and proacrosin, but not in inhibited acrosin, also occurred when spermatozoa were incubated under noncapacitating conditions for 8 h, indicating that capacitation may specifically cause the release of some acrosin inhibitor from human spermatozoa. It is concluded that, with the possible exception of hyaluronidase, the in vitro capacitation of human spermatozoa does not cause a major change in its acrosomal enzyme content so that these hydrolases are fully present before the acrosome reaction takes place during gamete fusion. Serum albumin appears to protect against the loss of some of these enzymes since the activity of several glycosidases was significantly reduced when the spermatozoa were incubated for 8 h in human serum albumin-free medium.