Fluorescence analysis of picoliter samples

β-hydroxybutyrate reduces blastocyst viability via trophectoderm-mediated metabolic aberrations in mice

STUDY QUESTION

What is the effect of the ketone β-hydroxybutyrate (βOHB) on preimplantation mouse embryo development, metabolism, epigenetics and post-transfer viability?

SUMMARY ANSWER

In vitro βOHB exposure at ketogenic diet (KD)-relevant serum concentrations significantly impaired preimplantation mouse embryo development, induced aberrant glycolytic metabolism and reduced post-transfer fetal viability in a sex-specific manner.

WHAT IS KNOWN ALREADY

A maternal KD in humans elevates gamete and offspring βOHB exposure during conception and gestation, and in rodents is associated with an increased time to pregnancy, and altered offspring organogenesis, post-natal growth and behaviour, suggesting a developmental programming effect. In vitro exposure to βOHB at supraphysiological concentrations (8–80 mM) perturbs preimplantation mouse embryo development.

STUDY DESIGN, SIZE, DURATION

A mouse model of embryo development and viability was utilized for this laboratory-based study. Embryo culture media were supplemented with βOHB at KD-relevant concentrations, and the developmental competence, physiology, epigenetic state and post-transfer viability of in vitro cultured βOHB-exposed embryos was assessed.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Mouse embryos were cultured in vitro with or without βOHB at concentrations representing serum levels during pregnancy (0.1 mM), standard diet consumption (0.25 mM), KD consumption (2 mM) and diabetic ketoacidosis (4 mM). The impact of βOHB exposure on embryo development (blastocyst formation rate, morphokinetics and blastocyst total, inner cell mass and trophectoderm (TE) cell number), physiology (redox state, βOHB metabolism, glycolytic metabolism), epigenetic state (histone 3 lysine 27 β-hydroxybutyrylation, H3K27bhb) and post-transfer viability (implantation rate, fetal and placental development) was assessed.

MAIN RESULTS AND THE ROLE OF CHANCE

All βOHB concentrations tested slowed embryo development (P < 0.05), and βOHB at KD-relevant serum levels (2 mM) delayed morphokinetic development, beginning at syngamy (P < 0.05). Compared with unexposed controls, βOHB exposure reduced blastocyst total and TE cell number (≥0.25 mM; P < 0.05), reduced blastocyst glucose consumption (2 mM; P < 0.01) and increased lactate production (0.25 mM; P < 0.05) and glycolytic flux (0.25 and 2 mM; P < 0.01). Consumption of βOHB by embryos, mediated via monocarboxylate transporters, was detected throughout preimplantation development. Supraphysiological (20 mM; P < 0.001), but not physiological (0.25–4 mM) βOHB elevated H3K27bhb levels. Preimplantation βOHB exposure at serum KD levels (2 mM) reduced post-transfer viability. Implantation and fetal development rates of βOHB-treated embryos were 50% lower than controls (P < 0.05), and resultant fetuses had a shorter crown-rump length (P < 0.01) and placental diameter (P < 0.05). A strong sex-specific effect of βOHB was detected, whereby female fetuses from βOHB-treated embryos weighed less (P < 0.05), had a shorter crown-rump length (P < 0.05), and tended to have accelerated ear development (P < 0.08) compared with female control fetuses.

LIMITATIONS, REASONS FOR CAUTION

This study only assessed embryo development, physiology and viability in a mouse model utilizing in vitro βOHB exposure; the impact of in vivo exposure was not assessed. The concentrations of βOHB utilized were modelled on blood/serum levels as the true oviduct and uterine concentrations are currently unknown.

WIDER IMPLICATIONS OF THE FINDINGS

These findings indicate that the development, physiology and viability of mouse embryos is detrimentally impacted by preimplantation exposure to βOHB within a physiological range. Maternal diets which increase βOHB levels, such as a KD, may affect preimplantation embryo development and may therefore impair subsequent viability and long-term health. Consequently, our initial observations warrant follow-up studies in larger human populations. Furthermore, analysis of βOHB concentrations within human and rodent oviduct and uterine fluid under different nutritional states is also required.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by the University of Melbourne and the Norma Hilda Schuster (nee Swift) Scholarship. The authors have no conflicts of interest.

TRIAL REGISTRATION NUMBER

N/A.

Noninvasive metabolic profiling using microfluidics for analysis of single preimplantation embryos

Noninvasive analysis of metabolism at the single cell level will have many applications in evaluating cellular physiology. One clinically relevant application would be to determine the metabolic activities of embryos produced through assisted reproduction. There is increasing evidence that embryos with greater developmental capacity have distinct metabolic profiles. One of the standard techniques for evaluating embryonic metabolism has been to evaluate consumption and production of several key energetic substrates (glucose, pyruvate, and lactate) using microfluorometric enzymatic assays. These assays are performed manually using constriction pipets, which greatly limits the utility of this system. Through multilayer soft-lithography, we have designed a microfluidic device that can perform these assays in an automated fashion. Following manual loading of samples and enzyme cocktail reagents, this system performs sample and enzyme cocktail aliquotting, mixing of reagents, data acquisition, and data analysis without operator intervention. Optimization of design and operating regimens has resulted in the ability to perform serial measurements of glucose, pyruvate, and lactate in triplicate with submicroliter sample volumes within 5 min. The current architecture allows for automated analysis of 10 samples and intermittent calibration over a 3 h period. Standard curves generated for each metabolite have correlation coefficients that routinely exceed 0.99. With the use of a standard epifluorescent microscope and CCD camera, linearity is obtained with metabolite concentrations in the low micromolar range (low femtomoles of total analyte). This system is inherently flexible, being easily adapted for any NAD(P)H-based assay and scaled up in terms of sample ports. Open source JAVA-based software allows for simple alterations in routine algorithms. Furthermore, this device can be used as a standalone device in which media samples are loaded or be integrated into microfluidic culture systems for in line, real time metabolic evaluation. With the improved throughput and flexibility of this system, many barriers to evaluating metabolism of embryos and single cells are eliminated. As a proof of principle, metabolic activities of single murine embryos were evaluated using this device.

Noninvasive metabolic assessment of single cells

Because metabolism is fundamental to the successful functioning of a cell, perturbations in metabolism can signal compromised viability. For the assessment of viability, analyses not only must be sensitive enough to be employed at the single cell level, but also noninvasive. Ultramicrofluorescence is a technique that fulfills both criteria. By using specially prepared constriction pipets, created on a microforge and calibrated with tritiated water, volumes in the nanoand picoliter range can be accurately delivered. A miniaturization of enzymatic analysis using a fluorescence microscope, with either photometer or charge-coupled device attachments, is then employed to quantitate biochemical reactions in such submicroliter volumes. Subsequently, samples of culture medium surrounding an individual cell can be taken and analyzed in order to determine the rate of utilization or production of various metabolites in real time.

A comparative analysis of metabolism and viability in porcine oocytes during in vitro maturation

The importance of oocyte quality cannot be overstated, because it impacts all subsequent events during development of the embryo, the fetus and even the resulting offspring. Oocyte metabolism plays a critical role in supporting developmental competence via multiple mechanisms. It is beginning to be understood that metabolic pathways not only affect cytoplasmic maturation but may control nuclear maturation as well. A complete understanding of the precise roles that metabolism plays in determining oocyte quality is crucial for developing efficient in vitro maturation systems to support acquisition of oocyte competence. To date, this pursuit has not been entirely successful. Work in our laboratory on porcine oocyte metabolism has elucidated some of the intricate control mechanisms at work within the oocyte, not only for energy production, but also encompassing progression of nuclear maturation, mitochondrial activity and distribution, and oxidative and ionic stresses. We hypothesize that by utilizing oocyte metabolic data, we can develop more appropriate in vitro maturation systems that result in increased oocyte and embryo developmental competence.

Noninvasive assessment of human embryo nutrient consumption as a measure of developmental potential

OBJECTIVE

To determine the relationship between blastocyst development and morphology and embryo metabolism.

DESIGN

Noninvasive assessment of carbohydrate uptake and ammonium production by individual embryos.

SETTING

Private assisted reproductive technology unit.

PATIENT(S)

Patients donated, with consent, cryopreserved pronucleate embryos and noncryopreserved blastocysts.

INTERVENTION(S)

Culture of 60 thawed pronucleate embryos in sequential media to the blastocyst stage with concomitant noninvasive analysis of embryo metabolism and analysis of 13 blastocysts from noncryopreserved embryos.

MAIN OUTCOME MEASURE(S)

Pyruvate and glucose consumption as well as blastocyst formation and quality.

RESULT(S)

Pyruvate and glucose uptakes on day 4 were significantly higher by embryos that went on to form blastocysts than by embryos that failed to develop to the blastocyst stage. Glucose uptakes were greatest in those blastocysts of highest grade, whereas pyruvate uptakes were similar irrespective of blastocyst grade, indicating that glucose is the more important nutrient for the human blastocyst. Among blastocysts of the same grade from the same patient, there was considerable spread of glucose consumption, indicating that glucose consumption may be of use in identifying blastocysts for transfer. Ammonium production by individual embryos was also measured, reflecting amino acid transamination and use by the human embryo.

CONCLUSION(S)

The ability to identify in culture the embryo with the highest developmental potential will facilitate the move to single-embryo transfers.

Carbohydrate uptake by quiescent and reactivated mouse blastocysts

The mouse, Mus musculus, can maintain blastocysts in embryonic diapause in the uterus while suckling young. This study used microfluorimetry to simultaneously examine glucose and pyruvate uptake by quiescent blastocysts, and at four hourly intervals after administration of the reactivating stimulus, oestradiol-17 beta. Following the non-invasive analysis of energy metabolism, blastocysts were incubated in colcemid (0.2 mg/ml), and mitotic activity determined. Mitoses and cell numbers in reactivated embryos increased significantly within 8 and 12 hours, respectively, after oestradiol-17 beta administration, compared to those of diapause (control) blastocysts (0.5 +/- 0.1 vs. 0.22 +/- 0.03 mitoses/embryo; P < 0.05, and 141.8 +/- 1.5 vs. 133.8 +/- 2.4 cells/ embryo; P < 0.05). Similarly, pyruvate uptake by reactivating blastocysts (9.3 +/- 1.1) was significantly higher than controls (5.8 +/- 0.8 pmol/embryo/hour; P < 0.05), within 4 hours of oestradiol-17 beta, but by 16 hours after oestradiol-17 beta administration, pyruvate uptake by reactivating blastocysts was no longer significantly different from the delayed controls. In contrast, significant differences in glucose uptake between the reactivated and control groups were not evident until 16 hours after oestradiol-17 beta (reactivating, 14.9 +/- 1.5; control, 10.6 +/- 1.7 pmol/embryo/hour; P < 0.05). These results demonstrate that pyruvate rather than glucose could supply the additional energy required during the first 12 hours of reactivation in the mouse, but from 16 hours after injection of the reactivating stimulus oestradiol-17 beta, glucose is the predominant energy source.

Oxygen consumption and energy metabolism of the early mouse embryo

Oxygen consumption of preimplantation and early postimplantation mouse embryos has been measured using a novel noninvasive ultramicrofluorescence technique, based on an oil-soluble, nontoxic quaternary benzoid compound pyrene, whose fluorescence is quenched in the presence of oxygen. Pyruvate and glucose consumption, lactate production, and glycogen formation from glucose were also measured. Preimplantation mouse embryos of the strain CBA/Ca x C57BL/6 were cultured in groups of 10-30 in 2 microliters of modified M2 medium containing 1 mmol l-1 glucose, 0 mmol l-1 lactate, and 0.33 mmol l-1 pyruvate, for between 4-6 hr. Day 6.5 and 7.5 embryos were cultured singly in 40 microliters M2 medium for between 2-3 hr. Oxygen consumption was detected at all stages of development, including, for the first time, in the early postimplantation embryo. Consumption remained relatively constant from zygote to morula stages before increasing in the blastocyst and day 6.5-7.5 stages. When expressed as QO2 (microliters/mg dry weight/hr), oxygen consumption was relatively constant from the one-cell to morula stages before increasing sharply at the blastocyst stage and declining to preblastocyst levels on days 6.5 and 7.5. Pyruvate was consumed during preimplantation stages, with glucose uptake undetectable until the blastocyst stage. Glucose was the main substrate consumed by the 6.5 and 7.5 day embryo. The proportions of glucose accounted for by lactate appearance were 81%, 86%, and 119% at blastocyst, day 6.5, and day 7.5 stages, respectively. The equivalent figures for glucose incorporated into glycogen were 10.36%, 0.21%, and 0.19%, respectively. The data are consistent with a switch from a metabolism dependent on aerobic respiration during early preimplantation stages to one dependent on both oxidative phosphorylation and aerobic glycolysis at the blastocyst stage, a pattern which is maintained on days 6.5 and 7.5. Our technique for measuring oxygen consumption may have diagnostic potential for selecting viable embryos for transfer following assisted conception techniques in man and domestic animals.

Effects of metabolic inhibitors on mouse preimplantation embryo development and the energy metabolism of isolated inner cell masses

The effects of two metabolic inhibitors, methyl palmoxirate (MP) and amino-oxyacetate (AOA), on mouse preimplantation embryo development and cell number, and inner cell mass. (ICM) cell metabolism have been examined. Two-cell embryos were cultured in media supplemented with either MP, which inhibits fatty acid oxidation, or AOA, which inhibits the transamination of glutamate into alpha-ketoglutarate. Embryos were scored for development daily. On day 5, expanded blastocysts were differentially labeled with fluorochromes to visualize TE and ICM cell nuclei, or the ICMs isolated by immunosurgery and their energy metabolism determined using microfluorometric methods. Embryos exposed to the two inhibitors developed into fully expanded blastocysts, although cell numbers of both the TE and ICM cells were significantly reduced compared to controls. The uptake of glucose in the presence of 1 mM MP or AOA did not differ from the controls, but less glucose was accountable for by lactate production. MP significantly reduced lactate production. In the presence of 4 mM AOA, the amount of glucose oxidized and the amount of lactate formed by ICMs were significantly reduced. The results indicate that the fuels used by isolated mouse ICMs vary in response to substrate availability and that fatty acids may be a potential energy source.

Role of glucose in mouse preimplantation embryo development

Mouse preimplantation embryos consume pyruvate preferentially during the early developmental stages, before glucose becomes the predominant energy substrate in the blastocyst. To investigate the importance of the switch to glucose utilization at the later developmental stages, mouse embryos from F1 hybrid mice (CBA/Ca x C57BL/6) were cultured from the one- and two-cell stages (22 and 46 h post hCG, respectively) for 5 days in a modified medium, M16, containing 0.33 mM pyruvate and 5 or 23 mM D + L-lactate, in the presence and absence of 1 mM glucose (M16 + G and M16 - G, respectively). Nutrient uptakes were also determined over this time. Some embryos cultured in M16 - G were transferred to M16 + G at 94 or 118 h post hCG. Embryos cultured from the two-cell stage in M16 + G exhibited the characteristic fall in pyruvate consumption between the morula and the blastocyst stage; those cultured from the two-cell stage in M16 - G compensated for the lack of glucose by consuming increasing amounts of pyruvate, from 2.78 pmol/embryo/h at 58 h post hCG to 5.21 pmol/embryo/h at 154 h post hCG. However, the percentage of embryos developing to the blastocyst stage, the hatching rate, and blastocyst cell numbers (50.6 +/- 2.5 [28] vs. 105 +/- 3.8 [37]) were all lower in this group. When exposed to glucose at 94 or 118 h post hCG, embryos cultured from the two-cell stage in M16 - G readily consumed glucose in preference to pyruvate, although the characteristic fall in pyruvate consumption was not observed. One-cell embryos cultured continuously in M16 - G were only able to develop to the morula stage, after which time they degenerated. In these embryos pyruvate was readily consumed between 22 and 94 h post hCG, before falling from 2.77 pmol/embryo/h at 83 h post hCG to 0.045 pmol/embryo/h at 130 h post hCG. Transfer of these embryos to M16 + G at 94 and 118 h post hCG did not support development to the hatching blastocyst stage.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of exogenous energy substrates in blastocoele fluid accumulation in the rat

Preimplantation mammalian development culminates in the formation of a fluid-filled cavity, the blastocoele, which is a prerequisite for successful implantation and further development. The blastocoele is enclosed by a single layer of polarised cells, the trophectoderm, which is the first epithelium formed in development. In embryos of the mouse and the rabbit, a basolaterally located Na+/K(+)-ATPase hydrolyses ATP to drive the vectorial transport of ions, which is responsible for the accumulation of blastocoele fluid. Using non-invasive assays of energy substrate consumption and blastocoele fluid accumulation, experiments were carried out on single preimplantation rat embryos, to establish: (1) the roles of the Na+/K(+)-ATPase and exogenous energy substrates, and (2) the relationship between the consumption and metabolism of energy substrates and fluid accumulation, during blastocoele cavity formation in this species. Ouabain 0.5 mM and energy-substrate-free medium both caused an inhibition in the number of embryos forming a blastocoele in culture, and also reduced the rate of fluid accumulation by day 5 blastocysts collapsed in cytochalasin-D and allowed to re-expand. Ouabain also reduced the consumption of glucose (but not pyruvate) and the production of lactate by re-expanding blastocysts. In the absence of the inhibitor, a direct relationship was seen between fluid accumulation and both glucose (but not pyruvate) consumption and lactate production. However, ouabain had no effect on intact, expanded blastocysts.(ABSTRACT TRUNCATED AT 250 WORDS)

Energy metabolism of the trophectoderm and inner cell mass of the mouse blastocyst

The two cell populations of the day 5 mouse blastocyst--the inner cell mass (ICM) and trophectoderm (TE)--were isolated by immunosurgery and TE biopsy. The uptake of glucose and pyruvate and the production of lactate were measured in the isolated cell populations and in single day 5 blastocysts, using a non-invasive ultramicrofluorescence technique. The number of cells in isolated ICMs, TE vesicles, and intact blastocysts were counted after differential labeling with fluorochromes. The uptake of glucose and the formation of lactate were approximately three and five times greater, respectively, in the ICM cells compared with the TE cells. All the glucose consumed by the ICM could be accounted for by lactate formation, whereas the value for TE was 55%. The uptake of pyruvate was low in both cell types. The results are consistent with the proposition that the TE acts as a transporting epithelium, sparing nutrients for metabolism by the ICM.

Glucose, pyruvate, and lactate concentrations in the blastocoel cavity of rat and mouse embryos

The concentrations of glucose, pyruvate, and lactate have been measured in the blastocoel fluid of single rat and mouse blastocysts, using the technique of micropuncture combined with an ultramicrofluorescence assay. When cultured in the presence of 5.55 mM glucose, 11.5-12.5 mM L-lactate and 0.25-0.33 mM pyruvate, concentrations in the blastocoel fluid of mouse and rat were 2.30 and 2.75 mM glucose, 14.6 and 19.6 mM L-lactate, and 0.13 and 0.50 mM pyruvate, respectively. When cultured in the presence of 1.0 mM glucose and 1.0 mM L-lactate, concentrations in the blastocoel fluid were 0.50 and 0.59 mM glucose and 2.22 and 3.70 mM L-lactate, respectively. These results suggest that (1) the blastocyst is capable of maintaining considerable concentration gradients of substrates across the trophectoderm, (2) the microenvironment of the blastocoel is adequately supplied with energy substrates for the development of the inner cell mass, and (3) the inner cell mass is capable of developing in both high and low glucose and lactate concentrations.

Early human embryo metabolism

Non-invasive microanalytical methods have been devised to study the energy metabolism of single human preimplantation embryos. Pyruvate, which is added routinely to all media used to culture human embryos, is consumed throughout the preimplantation period, with glucose assuming an increasing role at embryo compaction and blastocyst formation. All of the glucose consumed may be accounted for by the appearance of lactate in the incubation medium. The enzyme hexokinase may be involved in regulating this aerobic glycolysis. There is considerable indirect evidence for the utilisation of endogenous as opposed to exogenous energy substrates, the most likely candidate being protein. Information on early human embryo metabolism is likely to find application in a number of areas: these include the improvement of techniques for assisted human conception, notably in the selection of embryos for transfer following In Vitro Fertilisation; the diagnosis of genetic defects at the preimplantation stage; increased understanding of the causes of implantation failure and miscarriage, and the development of novel post-coital contraceptives.

Preliminary characterization of pyruvate uptake by one-cell ovine embryos

Pyruvate uptake was measured by a noninvasive method on a total of 59 individual one-cell ovine embryos. Uptake was measured over a 4-h period in Whitten's medium containing 1.0 mM glucose and either 0.0 mM, 0.33 mM, 0.67 mM or 1.0 mM sodium pyruvate. Glucose uptake was also assessed to determine the effect of altering pyruvate concentration on glucose uptake. Embryos incubated in medium containing 1.0 mM or 0.67 mM pyruvate showed average uptake values of 20.36 and 5.00 pmoles per embryo per 4 h, respectively. In contrast, when pyruvate concentrations were 0.0 mM and 0.33 mM, the embryos released an average of 24.60 and 10.40 pmoles pyruvate per embryo per 4 h. Uptake or release of pyruvate was found to vary linearly with concentration level. Increasing pyruvate concentration resulted in a significant decrease in glucose uptake.

Development of a noninvasive ultramicrofluorometric method for measuring net uptake of glutamine by single preimplantation mouse embryos

A noninvasive ultramicrofluorometric method for measuring net uptake of glutamine by single preimplantation mouse embryos is described. A linear relationship was found between fluorescence intensity of NADH produced and glutamine concentration (R2 = 0.985). Single embryos were placed in 20 nl drops of medium containing 0.5 mM glutamine, and the medium was sampled after 2 hr incubation at 37 degrees C. Changes in net glutamine uptake were determined in one-cell, two-cell, and eight-cell embryos and blastocysts incubated in medium containing no energy substrates (glucose, pyruvate, and lactate). The median glutamine uptake increased significantly from 0.480 and 0.270 pmoles/embryo/2 hr at the one-cell and two-cell stages, respectively, to 1.610 pmoles/embryo/2 hr at the blastocyst stage. Mean glutamine uptake was compared in the presence or absence of energy substrates at several developmental stages. A highly significant reduction of glutamine uptake in the presence of substrates was observed at the one-cell and two-cell stages of development. At the eight-cell stage, glutamine uptake was only marginally reduced in the presence of substrates, and no effect was found at the blastocyst stage. These data may partially explain the beneficial effect of glutamine on the culture of early mouse embryos through the two-cell block of development.

Use of ultramicrofluorometric methods for the study of single preimplantation embryos

Ultramicrofluorescence and video fluorescence microscopy are highly sensitive techniques for studying embryo metabolism and developmental potential. These techniques are attractive in that they allow one to study single embryos and thus to detect more subtle changes in embryo metabolism or intracellular pH which, using less sensitive techniques or batches of embryos, would likely go undetected. This is especially important in those species where few embryos can be recovered from a single mother. An additional advantage of ultramicrofluorescence techniques lies in their lack of invasiveness to the embryo under study. When measuring nutrient uptake, several measurements can be made on a single embryo that can then be placed back into culture for evaluation of its developmental potential. Alternatively, test embryos can be transferred back into a recipient for determination of embryo viability. This flexibility further illustrates the beneficial aspects of using ultramicrofluorescence techniques for studying embryo metabolism and development.

Assessment of embryo viability prior to transfer by the noninvasive measurement of glucose uptake

An ultramicrofluorometric technique was used to measure the uptake of glucose by single mouse blastocysts prior to transferring them into the uterus of pseudopregnant foster mothers. Of 50 blastocysts whose glucose uptake was measured prior to embryo transfer, 25 gave rise to male and 13 to female offspring. Twelve embryos failed to develop. The mean glucose uptake of these 12 embryos was significantly below that of those which developed successfully. Female embryos took up slightly more glucose than male embryos.

Pancreatic zymogen granules differ markedly in protein composition

The activities of both chymotrypsin and amylase in individual zymogen granules of rat pancreas were measured by means of micromanipulation and microfluorometric methods. The enzyme content and the ratio of amylase to chymotrypsin varied widely among granules taken from the same animal. These results are compatible with short-term nonparallel bulk secretion of the two enzymes through exocytosis. The distribution of each enzyme activity in a population of granules suggests quantal packaging of amylase and chymotrypsinogen into the granules.

Laser fluorimetry for capillary column liquid chromatography: high-sensitivity detection of derivatized biological compounds

A high-sensitivity, low-dead-volume laser-induced fluorescence detector has been designed for use in capillary column liquid chromatography. This detector exhibits femtogram detection limits and a linear dynamic range spanning five orders of magnitude. Derivatization schemes were utilized to add a fluorescent moiety to biologically important compounds such that their excitation maxima corresponded well with the output wavelength of the laser. Sample chromatograms are presented for picogram amounts of standard bile acid and steroid hormone derivatives. In addition, a high-efficiency separation of picogram amounts of the solvolyzable steroid hormones from human serum is illustrated.

Analysis of picogram quantities of protein in subnanoliter-size samples

The ability to measure protein concentration in subnanoliter volumes would be helpful in many biological studies. A microassay for measuring nanogram protein quantities in nanoliter-size samples and an ultramicroassay for measuring picogram quantities in picoliter samples were developed to measure lymphatic protein concentration. Aliquots of a sample solution were mixed with an o-phthalaldehyde mercaptoethanol reagent using micropipetting techniques. Reaction product fluorescence was measured using a modified Leitz MPV-1 microscope as a microfluorometer. Fluorescence varied linearly with albumin concentrations between 1 and 8 g/100 ml. A typical microassay measuring albumin standards at 0.0, 1.0, 2.0, and 4.0 g/100 ml yielded a linear regression of y = 207x + 60 (r = 0.99). Minimum detectable protein concentration was 0.125 g/100 ml. The SE for the albumin standards varied from 0.02 to 0.17 g/100 ml. An ultramicroassay measuring similar standards yielded a linear regression of y = 1180x + 109 (r = 0.96). Minimum detectable protein concentration was 0.028 g/100 ml. The SE for the standards varied from 0.01 to 0.32 g/100 ml.

Nucleotides in a single mammalian ovum or preimplantation embryo

ATP, ADP, and AMP have been measured jointly on a single mouse ovum or preimplantation embryo using an ultramicrofluorescence technique. The method uses the traditional approach of enzymatic analysis based on changes in the concentrations of nucleotide cofactors, but eliminates the need for enzymatic recycling. It permits the measurement of as little as 10 fmol, and may be adapted for numerous metabolites.

An NADH-coupled assay for femtogram or nanogram quantities of chymotrypsin

Chymotrypsin can be determined with an NADH-coupled assay. Hydrolysis of the substrate benzoyltyrosine ethyl ester is monitored by coupling the liberation of ethanol to the production of NADH and determining the NADH spectrophotometrically or fluorometrically. Nanogram quantities of chymotrypsin can be determined in milliliter volumes. With these microfluorescence methods this assay can be performed in a final volume of less than a nanoliter, allowing determination of femtogram quantities of chymotrypsin, the amount present in an individual zymogen granule.