Mutual cross-adaptation of the volatile steroid androstenone and a non-steroid perceptual analog

Self-and cross-adaptation are believed to result from stimulationof the same olfactory sensory channels. These adaptation phenomenawere studied after exposures to 5-androst-16-en-3-one (androstenone)and a synthetic perceptual analog (DMCMC), viz. a racemic mixtureof the isomers 4(R)-(4',4'-dimethyl-cyclohexyl)-2(R)-methylcyclohexanoneand 4(S)-(4',4'-dimethylcyclohexyl)-2(S)-methylcyclohexanone.In Experiment 1, six subjects very sensitive to androstenonereceived four randomized sequences of six concentrations offour ordants (androstenone, DMCMC, amyl acetate, and Galaxolide*;plus blanks) before and following adaptation to either androstenoneor DMCMC. Exposure to each odorant resulted in self-adaptation.Measures of stimulus intensity and identification thresholdrevealed reciprocal cross-adaptation between androstenone andDMCMC, but no cross-adaptation to amyl acetate or Galaxolide.The degree of cross-adaptation was asymmetric; adaptation toDMCMC resulted in more complete adaptation to androstenone thanvice versa. This asymmetry was apparently due to intensity differences;when stimuli were matched for intensity, the asymmetry disappeared(Experiment 2). These results demonstrate cross-adaptation forqualitatively similar, but not dissimilar, odors and suggestthat androstenone and its perceptual analog DMCMC share thesame sensory channels.     

Cross-adaptation of Sweaty-smelling 3-methyl-2- hexenoic Acid by a Structurally-similar, Pleasant-smelling Odorant

Cross-adaptation has been interpreted as a measure of the degreeto which odors share common sensory channels. How structuralsimilarity, in the absence of perceptual similarity, influencescross-adaptation is unknown. The present study assessed cross-adaptationby structurally similar, but perceptually different, odorants.Magnitude estimates for a 10:1 mixture of (E)- and (Z)-3-methyl-2-hexenoicacid (3M2H), a principal component of human underarm odor, decreasedfollowing adaptation to a mixture of (E)- and (Z)-ethyl estersof 3M2H (EE3M2H), which possess a pleasant, fruity odor. Cross-adaptationwas asymmetric; adaptation to 3M2H did not significantly affectthe perceived intensity of EE3M2H. By contrast, there was nosignificant cross-adaptation between 3M2H and the fruity-smellingethyl esters of its homologues, 3-methyl-2-octenoic acid (EE3M20)and 3-methyl-2-pentenoic acid (EE3M2P). Similarity ratings revealedno differences among the three ethyl esters in their perceptualsimilarity to 3M2H (i.e. all were rated equally dissimilar to3M2H). Molecular modeling studies revealed no difference inthe charge distribution of these molecules. Rather, differencesin the shape and size of the hydrophobic part of the moleculemay determine the extent of cross-adaptation. These resultsdemonstrate that structurally-similar, yet perceptually-distinct,odorants may cross-adapt and suggest that the extent of cross-adaptationmay be affected by the degree of structural, as well as perceptual,similarity. Chem. Senses 20: 401–411, 1995.     

On the trigeminal percept of androstenone and its implications on the rate of specific anosmia

Specific anosmia is a term that describes an inability to perceive a particular odorant in the context of an otherwise normal olfactory acuity. The most common example, for the odor of androstenone, has been ascribed a prevalence ranging from 2 to 45%. In two experiments we sought to determine whether this wide range could be explained by the difference in steroid concentrations used, and by the degree to which the trigeminal system contributes to perception of androstenone. Experiment 1 demonstrated that high concentrations of androstenone stimulated the trigeminal system, as indicated by electrophysiological recordings. Experiment 2 demonstrated that conscious detection of androstenone is possible based solely on the trigeminal system. Interestingly, detection seems to interact with olfactory acuity in that subjects with a low olfactory sensitivity to androstenone were better able to detect its trigeminal component. The agreement between conscious experience and behavioral discrimination was not well calibrated, in that subjects demonstrated a clear overconfidence in their abilities. Altogether, the current study suggests that androstenone is an odorant that produces a concentration-dependent degree of trigeminal stimulation. This trigeminal component explains the diversity of the reported prevalence of specific anosmia for androstenone and might have implications on future use of specific anosmia as a tool to understand odor processing.     

Dissociable codes of odor quality and odorant structure in human piriform cortex

The relationship between odorant structure and odor quality has been a focus of olfactory research for 100 years, although no systematic correlations are yet apparent. Animal studies suggest that topographical representations of odorant structure in olfactory bulb form the perceptual basis of odor quality. Whether central olfactory regions are similarly organized is unclear. Using an olfactory version of fMRI cross-adaptation, we measured neural responses in primary olfactory (piriform) cortex as subjects smelled pairs of odorants systematically differing in quality and molecular functional group (as one critical attribute of odorant structure). Our results indicate a double dissociation in piriform cortex, whereby posterior regions encode quality (but not structure) and anterior regions encode structure (but not quality). The presence of structure-based codes suggests fidelity of sensory information arising from olfactory bulb. In turn, quality-based codes are independent of any simple structural configuration, implying that synthetic mechanisms may underlie our experience of smell.     

Odor Perception and Beliefs about Risk

Although the perceptual response to environmental odors canbe quite variable, such variation has often been attributedto differences in individual sensitivity. An information-processinganalysis of odor perception, however, treats both the receptionand the subsequent evaluation of odor information as determinantsof the perceptual response. Two experiments investigated whethera factor that influenced the evaluation stage affected the judgementof odor quality and the degree of adaptation to the odor. Peoplewere surveyed in order to measure their tacit perceptions ofthe healthfulness or hazardousness of nine common olfactorystimuli, and the instructional context influenced quality perception.In a second experiment subjects were exposed to an ambient odorunder one of three different conditions, and odorant characterizationinfluenced the degree of adaptation to the odor. Subjects whowere led to believe the odor was a natural, healthy extractshowed adaptation; those told that the odor was potentiallyhazardous showed apparent sensitization; while those told thatthe odor was a common olfactory test odorant showed a mixedpattern: some exhibited adaptation, whereas others showed sensitization.However, detection thresholds obtained before and after exposureshowed adaptation effects that are characteristic of continuousexposure. These findings raise the possibility that cognitivefactors may be modulating the overall sensory perception ofodor exposure (i) for some individuals who exhibit extreme sensitivityto odors and (ii) in situations where adaptation to environmentalodors is expected but does not occur. Chem. Senses 21: 447–458,1996.     

Genetic variation of an odorant receptor OR7D4 and sensory perception of cooked meat containing androstenone

Although odour perception impacts food preferences, the effect of genotypic variation of odorant receptors (ORs) on the sensory perception of food is unclear. Human OR7D4 responds to androstenone, and genotypic variation in OR7D4 predicts variation in the perception of androstenone. Since androstenone is naturally present in meat derived from male pigs, we asked whether OR7D4 genotype correlates with either the ability to detect androstenone or the evaluation of cooked pork tainted with varying levels of androstenone within the naturally-occurring range. Consistent with previous findings, subjects with two copies of the functional OR7D4 RT variant were more sensitive to androstenone than subjects carrying a non-functional OR7D4 WM variant. When pork containing varying levels of androstenone was cooked and tested by sniffing and tasting, subjects with two copies of the RT variant tended to rate the androstenone-containing meat as less favourable than subjects carrying the WM variant. Our data is consistent with the idea that OR7D4 genotype predicts the sensory perception of meat containing androstenone and that genetic variation in an odorant receptor can alter food preferences.     

The effects of adaptation on the perception of similar and dissimilar odors

Adaptation among several odors was studied using a multiple-alternative,forced-choice procedure where six concentrations of each odorantand three blanks were presented in a random sequence beforeand after adaptation to each of the odors. Adaptation was expressedin terms of changes in both identification threshold and perceivedintensity. In the first experiment, Galaxolide® (a syntheticisochroman musk) and 5-androst-16-en-3-one (androstenone) showedno cross-adaptation in spite of sharing, for some people, amusky note. In the second experiment, Galaxolide and Thibetolide®(a synthetic macrocyclic musk) showed significant, but asymmetric,cross-adaptation. When Galaxolide was the adapting stimulus,shifts in indentification threshold and magnitude estimatesfor Thibetolide were not significantly different from thoseobtained for Galaxolide, a result consistent with the conclusionthat cross-adaptation was as effective as self-adaptation. WhenThibetolide was the adapting stimulus, shifts in identificationthreshold and magnitude estimates for Galaxolide were significantlyless than those obtained for Thibetolide, suggesting that cross-adaptationwas less effective. This result is puzzling given the perceptualsimilarity of these two musky compounds: subjects did not distinguishbetween them during random presentation. The distinct odorsof amyl acetate (banana) and d-limonene (orange) showed no cross-adaptationin either experiment.     

Perceptual Grouping Enhances Visual Plasticity

Visual perceptual learning, a manifestation of neural plasticity, refers to improvements in performance on a visual task achieved by training. Attention is known to play an important role in perceptual learning, given that the observer''s discriminative ability improves only for those stimulus feature that are attended. However, the distribution of attention can be severely constrained by perceptual grouping, a process whereby the visual system organizes the initial retinal input into candidate objects. Taken together, these two pieces of evidence suggest the interesting possibility that perceptual grouping might also affect perceptual learning, either directly or via attentional mechanisms. To address this issue, we conducted two experiments. During the training phase, participants attended to the contrast of the task-relevant stimulus (oriented grating), while two similar task-irrelevant stimuli were presented in the adjacent positions. One of the two flanking stimuli was perceptually grouped with the attended stimulus as a consequence of its similar orientation (Experiment 1) or because it was part of the same perceptual object (Experiment 2). A test phase followed the training phase at each location. Compared to the task-irrelevant no-grouping stimulus, orientation discrimination improved at the attended location. Critically, a perceptual learning effect equivalent to the one observed for the attended location also emerged for the task-irrelevant grouping stimulus, indicating that perceptual grouping induced a transfer of learning to the stimulus (or feature) being perceptually grouped with the task-relevant one. Our findings indicate that no voluntary effort to direct attention to the grouping stimulus or feature is necessary to enhance visual plasticity.     

Pemenone and Androstenone do not Cross-adapt Reciprocally

Explorations of the qualitative and quantitative differencesbetween the odors of pemenone (PEM), androstenone (AND) andisovaleric acid (IVA) show that they share a number of commonperceptual characteristics. Among these are similarities intheir odor quality and relative intensity ratings. PEM is alsoan efficient cross-adaptor and modulator of a subject's ANDsensitivity. Here we evaluate the reciprocal efficacy of ANDadaptation to alter the perceived intensity and quality of PEM,IVA and AND. Twenty-three people, including both those osmicand allosmic (n = 11) for the putrid odor quality of PEM, weretested. Following training in odor quality and intensity ratingtechniques, subjects sampled a selected substance for 2 minto obtain adaptation and then reported quality and intensityratings for the three test stimuli. There was significant self-adaptationby PEM and IVA in all subjects, but self-adaptation by AND wasonly observed in the PEM-osmic subjects. AND did not cross-adaptPEM or IVA to any significant extent. Collectively, these resultscontrast with our earlier study in which PEM was an efficientcross-adaptor of AND. Here, AND was no more efficient than thecontrol as an adapting substance for PEM, despite significantself-adaptation of PEM by itself. This lack of reciprocity inthe effectiveness of PEM and AND as cross-adapters is not relatedto differences in odor intensity, as the PEM and AND concentrationswere adjusted for each subject to elicit comparable intensityreports. These results support the notion that PEM, AND andIVA share certain perceptual characteristics, but interact differentiallywith three or more sets of perceptual channels that are nowthought to result in a putrid odor quality. Chem. Senses 21:711–717, 1996.     

A Spectroscopic Mechanism for Primary Olfactory Reception

A novel theory of primary olfactory reception is described.It proposes that olfactory receptors respond not to the shapeof the molecules but to their vibrations. It differs from previousvibrational theories (Dyson, Wright) in providing a detailedand plausible mechanism for biological transduction of molecularvibrations: inelastic electron tunnelling. Elements of the tunnellingspectroscope are identified in putative olfactory receptorsand their associated G-protein. Means of calculating electrontunnelling spectra of odorant molecules are described. Severalexamples are given of correlations between tunnelling spectrumand odour in structurally unrelated molecules. As predicted,molecules of very similar shape but differing in vibrationssmell different. The most striking instance is that of pureacetophenone and its fully deuterated analogue acetophenone-d₈,which smell different despite being identical in structure.This fact cannot, it seems, be explained by structure-basedtheories of odour. The evidence presented here suggests insteadthat olfaction, like colour vision and hearing, is a spectralsense. Chem. Senses 21: 773–791, 1996.     

Adaptation and cross-adaptation to odor stimulation of olfactory receptors in the tiger salamander

We have used the effects of self- and cross-adaptation on the unitary responses of olfactory receptors of the tiger salamander to odor stimulation to investigate the stimulus-specific components of these responses and to provide information about the cross-cell variations in the numbers and numbers of types of constitutent receptive sites. An olfactometer delivered sequential odorous pulses, either juxtaposed or separated by a variable time delay. We used four pairs of odorants judged to be similar within a given pair. The unitary response to the test stimulation relative to that of the conditioning stimulation varied from being unchanged to being completely eliminated. We sometimes observed substantial poststimulus increases in the firing rate following stimulation with juxtaposed odorous pulse. Except in the case of one odorant pair, cross-adaptation occurred both with juxtaposed pulses and with pulses separated in time. With the methyl butyrate/ethyl butyrate odorant pair, however, statistically significant cross-adaptation appeared only with juxtaposed pulses. We propose a simple model to aid in explaining these phenomena. The experimental observations in conjunction with this model are used to obtain estimates of the maximal and minimal number of receptive site types available for interaction with the chosen odorants.     

Taste Responses to Binary Mixtures of Amino Acids in the sea catfish, Arius felis

In vivo electrophysiological recordings in the sea catfish,Arius felis, showed that the magnitude of the integrated facialtaste responses to binary mixtures of amino acids was predictablewith knowledge obtained from previous cross-adaptation studiesof the relative independence of the respective binding sitesof the component stimuli. Each component from which equal aliquotswere drawn to form the mixtures was adjusted in concentrationto provide for approximately equal reponse magnitudes. The magnitudeof the taste responses to binary mixtures whose component aminoacids showed minimal cross-adaptation was significantly greaterthan that to binary mixtures whose components exhibited considerablecross-reactivity. There was no evidence for mixture suppression.The relative magnitude of the taste responses in the sea catfishto stimulus mixtures is similar to that previously reportedfor olfactory receptor responses in the freshwater channel catfishand chorda tympani taste responses in the hamster. Chem. Senses21: 45–53, 1996.     

Learning to smell the roses: experience-dependent neural plasticity in human piriform and orbitofrontal cortices

It is widely presumed that odor quality is a direct outcome of odorant structure, but human studies indicate that molecular knowledge of an odorant is not always sufficient to predict odor quality. Indeed, the same olfactory input may generate different odor percepts depending on prior learning and experience. Combining functional magnetic resonance imaging with an olfactory paradigm of perceptual learning, we examined how sensory experience modifies odor perception and odor quality coding in the human brain. Prolonged exposure to a target odorant enhanced perceptual differentiation for odorants related in odor quality or functional group, an effect that was paralleled by learning-induced response increases in piriform cortex and orbitofrontal cortex (OFC). Critically, the magnitude of OFC activation predicted subsequent improvement in behavioral differentiation. Our findings suggest that neural representations of odor quality can be rapidly updated through mere perceptual experience, a mechanism that may underlie the development of odor perception.     

Olfactory discrimination of aliphatic odorants at 1 ppm: too easy for CD-1 mice to show odor structure–activity relationships?

Using an operant conditioning paradigm we tested the ability of CD-1 mice to discriminate between 25 odorants comprising members of five homologous series of aliphatic odorants (C4-C8) presented at a gas phase concentration of 1 ppm. We found (a) that all mice significantly discriminated between all 50 stimulus pairs that involved odorants sharing the same functional group, but differing in carbon chain length, as well as between all 50 stimulus pairs that involved odorants sharing the same carbon chain length but differing in functional group, (b) a significant negative correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length with the acetic esters and the 2-ketones, but not with the 1-alcohols, n-aldehydes, and n-carboxylic acids tested, and (c) that odorant pairs differing in functional group were significantly better discriminated than odorant pairs differing in carbon chain length. These findings demonstrate that CD-1 mice have excellent discrimination ability for structurally related aliphatic odorants, that correlations between discrimination performance and structural similarity of odorants are odorant class-specific rather than a general phenomenon, and that both carbon chain length and type of functional group play an important role for odor quality coding in mice.     

Absence of HLA association or linkage for variations in sensitivity to the odor of androstenone

Sensitivity to the odor of 5-androst-16-en-3-one (androstenone), a testosterone metabolite, shows wide variations among unrelated individuals. Analysis of correlations in sensitivity between monozygotic twin pairs, dizygotic twin pairs, and nontwin sib pairs now shows that at least a portion of this variation is genetically determined. However, although data from some mouse studies have suggested a relationship between olfaction and the murine histocompatibility system (H-2), we were unable to demonstrate any role of the human HLA system in explaining the wide individual variations in human sensitivity to androstenone. An additional analysis of HLA antigens among 61 human mating pairs also provided no evidence that HLA phenotypes play a role in human mating preference. These data fail to support a role for the human HLA system in the recognition of an odorant of potential biological significance.     

Just noticeable differences in component concentrations modify the odor quality of a blending mixture

The odors we perceive are mainly the result of mixtures of odorants that, however, are commonly perceived as single undivided entities; nevertheless, the processes involved remain poorly explored. It has been recently reported that perceptual blending based on configural olfactory processing can cause odorant mixtures to give rise to an emergent odor not present in the components. The present study examined whether specific component proportions are required to elicit an emergent odor. Starting from the composition of a ternary target mixture in which an emergent pineapple odor was perceived, 4 concentration levels of each component were chosen to elicit just noticeable differences (JNDs). Each combination of levels was used to design sample mixtures. Fifteen subjects evaluated the intensity, typicality, and pleasantness of each sample mixture against the target mixture in a paired-comparison protocol. Statistical modeling showed that a variation of less than 1 JND in one of the components was sufficient to induce a significant decrease in pineapple odor typicality in the ternary mixture. This finding confirms previous findings on perceptual blending in simple odorant mixtures and underscores the human ability to discriminate between odor percepts induced by mixtures including very similar odorant proportions.     

Perceptual learning in an appetitive Pavlovian procedure: Analysis of the effectiveness of the common element

Non-reinforced preexposure to two stimuli often enhances discrimination between them. Analyses of this perceptual learning phenomenon have mainly focused on the role played by the distinctive stimulus features; this study examined the contribution of the non-distinctive common elements. A standard appetitive Pavlovian procedure was used. Rats received two different schedules of exposure - alternated or blocked - to two compound auditory stimuli, AX and BX. In Experiment 1 a generalization test to BX that followed conditioning to AX showed that animals responded less, and hence discriminated better, following alternated exposure, thus extending the generality of this perceptual learning effect to standard appetitive Pavlovian procedures. The degree to which the common element X was mediating this effect was explored in the next three experiments. Experiment 2 assessed the effectiveness of X following conditioning to AX. Experiment 3 explored X's effectiveness throughout extensive conditioning to X. Experiment 4 tested the ability of X to overshadow a novel stimulus Y. The results were consistent with the suggestion that alternated preexposure can reduce the relative effectiveness of the common element.     

Mutation spectra in Salmonella of analogues of MX: implications of chemical structure for mutational mechanisms

We determined the mutation spectra in Salmonella of four chlorinated butenoic acid analogues (BA-1 through BA-4) of the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) and compared the results with those generated previously by us for MX and a related compound, MCF. We then considered relationships between the properties of mutagenic potency and mutational specificity for these six chlorinated butenoic acid analogues. In TA98, the three most potent mutagens, BA-3, BA-4, MX, and the organic extract, all induced large percentages of complex frameshifts (33-67%), which distinguish these agents from any other class of compound studied previously. In TA100, which has only GC sites for mutation recovery, >71% of the mutations induced by all of the agents were GC-->TA transversions. The availability of both GC and TA sites for mutation in TA104 resulted in greater distinctions in mutational specificity than in TA100. MX targeted GC sites almost exclusively (98%); the structurally similar BA-4 and BA-2 produced mutations at similar frequencies at both GC and AT sites; and the structurally similar BA-3 and BA-1 induced most mutations at AT sites (69%). Thus, large variations in structural properties influencing relative mutagenic potency appeared to be distinct from the more localized similar structural features influencing mutagenic specificity in TA104. Among a set of physicochemical properties examined for the six butenoic acids, a significant correlation was found between pK(a) and mutagenic potency in TA100, even when the unionized fraction of the activity dose was considered. In addition, a correlation in CLOGP for BA-1 to BA-4 suggested a role for bioavailability in determining mutagenic potency. These results illustrate the potential value of structural analyses for exploring the relationship between chemical structure and mutational mechanisms. To our knowledge, this is the first study in which such analyses have been applied to structural analogues for which both mutagenic potency and mutation spectra date were available.     

A comparison of the discriminatory ability and sensitivity of the trigeminal and olfactory systems to chemical stimuli in the tiger salamander

Summary Trigeminal receptors can respond to a wide variety of chemical stimuli, but it is unknown whether these receptors mediate discrimination between chemical stimuli matched for equal perceptual intensity. The present electrophysiological and behavioral experiments address this issue using tiger salamanders, Ambystoma tigrinum, and four compounds (amyl acetate, cyclohexanone, butanol, and d-limonene). In addition, the relative sensitivities of the trigeminaland olfactory systems to these compounds are compared. In electrophysiological cross-adaptation experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), there was complete cross adaptation such that only concentrations above the background (crossa-dapting) stimulus concentration elicited responses, suggesting that chemical stimuli may stimulate trigeminal receptors nonspecifically. In behavioral experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), only animals with intact olfactory nerves could discriminate between perceptually equivalent concentrations, that is concentrations that elicited the same level of responding. Both electrophysiologically and behaviorally, the trigeminal system exhibited higher thresholds than the olfactory system. We conclude that trigeminal chemoreceptors, at least in salamanders, are unable to discriminate between these two pairs of compounds when matched for equal perceptual intensity, and that trigeminal chemoreceptors are less sensitive than olfactory receptors.Abbreviations AA amyl acetate - CH cyclohexanone - LI d-limonene - BU butanol - EOG electro-olfactogram - ISI interstim-ulus interval - ONX olfactory nerve cut - ppm parts per million (1 l of compound in vapor phase/1l of air=1 ppm)     

Investigation of Testosterone,Androstenone, and Estradiol Metabolism in HepG2 Cells and Primary Culture Pig Hepatocytes and Their Effects on 17βHSD7 Gene Expression

Steroid metabolism is important in various species. The accumulation of androgen metabolite, androstenone, in pig adipose tissue is negatively associated with pork flavor, odour and makes the meat unfit for human consumption. The 17β-hydroxysteroid dehydrogenase type 7 (17βHSD7) expressed abundantly in porcine liver, and it was previously suggested to be associated with androstenone levels. Understanding the enzymes and metabolic pathways responsible for androstenone as well as other steroids metabolism is important for improving the meat quality. At the same time, metabolism of steroids is known to be species- and tissue-specific. Therefore it is important to investigate between-species variations in the hepatic steroid metabolism and to elucidate the role of 17βHSD7 in this process. Here we used an effective methodological approach, liquid chromatography coupled with mass spectrometry, to investigate species-specific metabolism of androstenone, testosterone and beta-estradiol in HepG2 cell line, and pig cultured hepatocytes. Species- and concentration-depended effect of steroids on 17βHSD7 gene expression was also investigated. It was demonstrated that the investigated steroids can regulate the 17βHSD7 gene expression in HepG2 and primary cultured porcine hepatocytes in a concentration-dependent and species-dependent pattern. Investigation of steroid metabolites demonstrated that androstenone formed a 3′-hydroxy compound 3β-hydroxy-5α-androst-16-ene. Testosterone was metabolized to 4-androstene-3,17-dione. Estrone was found as the metabolite for β-estradiol. Inhibition study with 17βHSD inhibitor apigenin showed that apigenin didn’t affect androstenone metabolism. Apigenin at high concentration (50 µM) tends to inhibit testosterone metabolism but this inhibition effect was negligible. Beta-estradiol metabolism was notably inhibited with apigenin at high concentration. The study also established that the level of testosterone and β-estradiol metabolites was markedly increased after co-incubation with high concentration of apigenin. This study established that 17βHSD7 is not the key enzyme responsible for androstenone and testosterone metabolism in porcine liver cells.