Bio-Organic Mechanisms II: Chemoreception Lloyd N. Ferguson California State University-Los Angeles. Los Angeles, CA 90032
Guest molecules react with the normal biolocical constitnen1.s in a t least six ways: (1) covalent hond formation, such as that done hy the alkylating anticancer drugs and the carcinogenic aromat,ic hydrocarbons; (2) chelation ( I ) , the way in which dithiols interact in the treatment of heavy metal poisoning; (3) intercalation, the way proposed for certain cancer drugs such as adriamycin and the antibacterial acridine drugs; (4) antimetabolite action, which is really more than a reaction, heing the therapeutic process exhihited hy the antibacterial sulfa drugs and some of the anticancer drugs such as 5-fluorouracyl and methotrexate; (5) charge-transfer complexion, for which most evidence is circumstantial, i.e., empirical correlations; and (6) chemoreception, the topic of this article. Generalizations A drug has been defined as a substance which in relatively small anantitv is able to elicit a resnonse from livine tissue. The notion ofg drug molecule interacting with a specific site on a cellular surface, i.e.. a biorecevtor. mav have heen suegested first by Langley ( 2 )although the term was introduced independently by Ehrlich (3).A drug receptor site is conceived as a relatively small region of a tissue constituent, which may he a functional part of a cell membrane or a cellular macromolecule such as an enzyme, a protein, or a nucleic acid. These receptors are visualized as havinz structural features which interact in a complementary manner with the foreign molecule (drug, substrate, agonist) to initiate a chain of events leading to an ohservable biological response. Normally, the suhstrate-receptor binding involves H-bonding and dipole-dipole, van der Waals, and polarization forces rather than covalent bonding. From SAR studies, early investigators compared the drug to a key that fits into a biological lock to exert its effect. Later observations, however, strongly support the idea of flexible receptors. The fit of a drug onto a receptor is rarely an allor-none situation. A molecule with a close match, called an agonist, produces a strong hiological response. Some compounds are strongly hound to a receptor site hut produce little or no biological response. These molecules, called antagonists, lack attachment to the recentor a t certain critical noints. Many drug effects take place a t hormone or neurotransmitter receptors or with enzymes, but rarely can the receptors he isolated. Most receptors are firmly attached to cell membranes. The rationale for the concevt of hioreceotors is
there is a stereospecificity wheGein one stermisomer is active, hut its enantiomer is inactive. For example, (+)-acetvlP-methylcholine has about 230 times the muicarinic activity of its enantiomer: manv D-amino acids are sweet whereas the L-isomers are not; (+)-norhomoepinephrine is 160 times more active as a pressor than (-)-norhomoepinephrine; (+Iascorbic acid has good antiscarhutic properties whereas the (-) isomer is inactive; (-)-epinephrine is 12-15 times more active as a vasoconstrictor than its mirror analog; (-1-hyo456
Journal of Chemical Education
scyamine is 15-20 times as potent a mydriatic as (+)-hyoscyamine. These differences may be accounted for in several ways. Differences in distribution, owing to differential diastereoisomer formation along the route to the receptor site, or stereospecific interaction with the receptor site are two possibilities. In the latter case, there are a t least three matching points hetween the active isomer and the receptor site hut one of these is missing for the inactive isomer.
Receptor
Drug-receptor fit
Lack of drugreceptor fit
On the other hand, both (+) and (-) isomers of cocaine are equipotent as a local anesthetic and hoth enantiomers of chloroquine are equally effective antimalarial drugs. A possible explanation is that no asymmetrical centers are involved in the drug-receptor interaction or that such interaction requires only one or two points of contact and not three.
I.
T h t distance hetwern pepride bond-. in an tmcndvd prutein ~ . l u i n X I \ . I t i\ noticeable rhar thr di-tan,.~hetween kev functional groups for several types of bioactivity is close to this distance or a whole number multiple of it. For example, virtually all sweet molecules have a molecular grouping designated H-A,B in which H-A and B are a proton-donor and a proton-acceptor involved in reciprocal H-bonding with the taste receptor ( 4 ) .A and B are separated by a distance of 3-4 A. A separation between amide C=O group and nitrogen atom in many cholinergic blocking agents is 7.2 A (2 X 3.6) ( 5 ) .
The distance between H-bonding for maximum activity in estrogenic compounds is 14.5 A (4 X 3.6) ( 6 ) .
Part I appeared on page 46 of the January 1980 issue of JOURNAL.
THIS
14.5 A Diethylntilbestrol
a stereospecific size and shape and which apparently determines the intensity of sweetness. Studies on various classes of molecules have discerned the following information about the sweet taste receptor(s). The H-A, B fragment should have a length of 3-4 and the hydrophobic portion about 5-7 A.
The distance between quaternary nitrogens of curare-like drugs is 14.5 A (7). t
-
+
MerN~lCHd~o~NMei
T h e corresponding spacing for two turns of the a-helical structure common to proteins is 5.5 A. This distance is found between required functional groups of many drugs. For example, the moiety
occurs in a variety of local anesthetics, adrenergic hlocking agents, antihistamines, and others (8).
For illustration, a sweet taste is sharply dependent upon the size and shape of the hydrocarbon fragment of sulfamates, R-NHS03Na (Table 1).This accounts for the observation that compounds I1 and 111in the table are sweet whereas I and IV are not; XII-XIV are sweet compared to nonsweet XI; XV is sweet compared to nonsweet XVI; and XI11 is sweet whereas XVII is not. Because van der Waals forces are of short range, hydrophobic bonding will be weak unless there is a good complementary "fit" between receptor and agonist. For dipeptides (XX), sweetness is observed when R2 has a length of 42-8.8 A,a volume greater than 30 A3, and has a maximum hulkiness a t 2-5 A from the CH carbon (12).
0
0
+
r-N,H, II -'O~ FH-C-N-CH
I
/&,
,
,C-CH2
I
ti
0
Antispasmodic
C ' O,
Benadryl (antihistamine)
The receptor site is probably flexible and altered in shave ~III.IIU ~II pc,Iar~nttam hr the .ubstr;rte, o r thew ctn~ld1,esevr;al , receptori i,idiiierwu sizes m(l shnpra. For t x a ~ n w l cstwral sweet families correspond to the substrate in XXI Novaeaine (local anesthetic)
The receptor site concept of chemoreception offers a logical explanation for many experimental SAR results. Currently, a vast amount of research on chemoreception is being pursued, generally along four approaches (9): (1)Translation of SAR results into mapping of receptor sites. (2) Study of the mechanisms hy whichthese sit& interact with suhs&ates. (3) Characterization of receptors ( 1 0 ) . (4) Pursuit of the critical steps in the chain of events after drug-receptor complexation which leads to the biological response. Our interest here overlaps the first two lines of investigation. The usual types of SAR studies provide empirical information for elucidating the topography of receptor sites. One can probe the nature of the substrate-receptor interaction by an inspection of the most active molecules, from a standpoint of their physiochemical properties, charge-density distributions, and stereochemistry. Two popular examples will be discussed here, one involving one of the chemical senses and the other, a type of psychoactive drug. The Sweet Taste Receptor
In the case of the sense of taste, only salts are salty, only acids are sour, hut a wide variety of structures are sweet, hitter, or tasteless. Most research in this area has been done on the sweet sense (11). It is generally agreed that the initial act for a taste sensation occurs on microvilli vrotrudina from taste-bud wores on the tongue and that the-process i;quick, reversible, and of low energy. A prevalent view is that it involves a stereospecific adsorption on the receptor site, including H-bonding and van der Waals and polarization forces. Shallenberger was the first to show convincingly that virtually all sweet compounds have a molecular fragment designated H-A, B in which H-A and B are proton-donor and proton-acceptor in H-bonding partnership with the receptor site. Associated with the H-A, B moiety in the sweet molecule is a hydrophobic segment with
XXI
Fine Aromatics ( 14):
Z X Y
= NH2, OH = 0.p-Uireeting
groups. e 4 . . alkvl, allroxyl, halogen. Z = mdireeting groups, e x . . NO,, CN. CF, Examples:
In each case, interchange of X and Y destroys the sweet taste. Dihydrochalcones ( 15)
O.H.O G
=
Elhamnose sugar, H0,S-(CHJ-, HO,DCHr
or
The sweet taste is absent when OH and OCH? exchange positions, or when the OCH, or OH are f l a k e d by another OH group.
Volume 58
Number 6
June 1981
457
Taste of Some N-alkylsulfarnates. R-NHS0,Na Compound NO.
R length,
A
Tastes
-
-CH&H2CH3
4.0 5.2
111 IV
-CH2CHtCH2CH3
6.4
-(CH&CHa
7.6
V
-CH(CHII~
VI VII Vlll
-CH2-CH(CH&
3.6
R
I
-CH2CHs
II
-CH,-CH(CH3)-CHz-CH,
-CH2CH2-CH(CH& -CH-CH2CH2-CH(CH& -cHrC(CHds cyclobutyl cyclapentyl cy~lohe~yl cycloheptyl cyclapentylmethyl cyclohexylrnethyl 4-methylcyclohexyl 4CHs)s
IX X XI XI1 Xlll XIV
XV XVI XVll XVlll a
( 13).
50 6.4 6.2
7.4 5.2 4.8
5.0 6.0 6.3 6.0 7.1 7.1
+ + -
+ +
+
-
+ + + + + -
+ = swaet
- = not sweat
FhyIloduIcins. DihyaCoisocouman'm, and F b v a m ( 16, 17):
Psychoactive Drugs .9 wrking chinition o i a psychoac~ivedrug is m e that 31ters 1)rhavior through its ittudificatiun of brain tunction. It is convenient to catrgorize s ~ w hp~ychoncriuedrugs un the basis of their most pnmtnent Iwhaviornl effect, eve!) though it is rerognized that varying duses of it ilruc may each pronwte different behavior. For inirnnce, alcolwl in small amounts is an excitant but is a depressant i t higher doses. One classification of drugs that alter mood or hehavior, including pain, is (18) Sedative-hypnotics (e.g., barbiturates benzodiazepines). Stimulants and convulsants (e.g., amphetamines). Narcotic analeesics (e.e.. ooiates such as morohine).
SAR studies have been carried out for each of these classes of psychoactive drugs. We will take up only one of these, namely, the narcotic analgesics, the most powerful of the ~ a i killer n drugs. However. we will first orovide a settine for discussion of &e opiates by'presenting some of the receni phenomenal discoveries (19) in brain chemistry (20). Two million Americans suffer from schizophrenia and another 12 million are clinicallv deoressed. As little as 25 vears ago, there was no effective ireaiment for these peopld. Although man has for centuries used certain olants to oroduce astuni,hing brh;~\,ioraleriects, i t is m 1 y i n i h r 1 ~ feu ~ years 1 that there has hem availnblr n sophisticated treatment for I he various types of psychosis such as depression, schizophrenia with its relentless voices and delusions, mania, etc. Most of 458
Journal of Chemical Education
the leads to present-day drugs were found by serendipity, but recent discoveries in brain chemistry are providing a more rational search for new psychotropic drugs (19). A cautiously used working hypothesis in this field is that mental illness results from an imbalance of neurotransmitters in the brain. Neurotransmitters are chemicals, such as the biogenic amines, dopamine, norepinephrine, and serotonin, that carry messages from one nerve cell (neuron) to the next and to nerve sites in the brain. They act by crossing the gap (synapse) between two neurons and binding to specific receptor sites on the adjacent neuron. Certain conditions will allow the releasing neuron to reahsorb the neurotransmitters. The revolution in psychoactive drugs began in the early 1950's when antischizophrenic drugs were found unexpectedly.
I
H
seraton&
Dopamine
Norepinegbrine (NE)
The strongly sedating drug chlorpromazine was used to calm patients prior to surgery. When tried as a tranquilizer on mental patients, not only did it calm them but also abated some of their symptoms. In the 60's, it was found that chlorpromazine and other antischizophrenic drugs increase the amount of dopamine in rat brains, and it was proposed that the drugs bind to the dopamine receptors to block its transmitting action. This hypothesis was not confirmed until the 70's.
Work from several lnborau)rrt~.;showed that there are a t least two 1v11vsof dopamine recevtors. One tvoe, .. . Dl.. is in the caudate n&eus that., upon exposure to dopamine, releases the enzyme adenylate cyclase, an enzyme that catalyzes the synthesis of cyclic AMP. The other type, D2, is on pituitary cells, that respond to dopamine by ceasing to make the hormone prolactiu. When a drug blocks D2 receptors, the pituitary cells secrete excessive amounts of prolactin which leads to menstrual abnormalities. Some antischizophrenic drugs block both types of action whereas others block only one. This provides drug researchers two types of assays for screening antischizophrenic drugs. An additional develo~mentin this area was use of radiotagging of substances to distinguish general from specific hindine of drugs. After exDosure of cells to a hiehlv .. . radioactive drug, the ctlli are u.nshed thur~~uyhly. which removes all but the speciiiralls hound druz. This rruuires much less ot rhr test sample, eliminates sacrificing hundreds of rats, and one can screen selectively for antianxiety, antidepressive, or antimania drugs. Also, it can lead to drugs which produce a minimum of side effects. For instance, some of the drugs also hind to a class of receptors, called muscarinic receptors, for the neurotransmitter acetylcholine. It has been found that the stronger the binding of drugs to muscarinic receptors, the less is the Parkinsonian side effect. Other serious side effects of antischizoohrenic dmes are hlwd pressure lowering and sedation. 1t w& found thacthese side effects are related to a class of receotors. . . called a-receorurs, for the neurotranimirter norepinep11r:ne. Drug.; that have a high ratio ot affinitv for (I-receptors to d o ~ a m i n ereceptors tend t o cause loweied blood piessure andsedation. Thus, in the search for new antischizophrenic drugs,
pharmaceutical chemists screen for compounds that hind well to dopamine receptors as well as to muscarinic receptors but poorly to a-receptors. Attention is also given to dopamine receptors linked to the release of cyclic AMP hut not prolactin. Going hack to the 50's again, it was observed that the antihypertension drug reserpine, which also causes depression, depletes the hrain of the neurotransmitters serotonin and norepinephrine. On the other hand, the antituhercular drug, iproniazid, which is an excitant, increases the levels of these two neurotransmitters by preventing an enzyme, monoamine oxidase (MAO),from metabolizing them. This led to the belief that deoression is associated with a lack of serotonin and norepinephrine in the brain. However, it was later found that some antide~ressants.the tricvclics. do not chanee the levels of serotoninand norkpineph;ine in the hrain. ~ p ~ a r e n t l y , after these amines have relayed their messaae to the next
neurotransmitter y-aminohutyric acid (GABA) receptor. The addition of GABA increases henzodiazepine binding. Consequently, it appears that there is a large number of neurotransmitters in the hrain (over a dozen have been observed) and that altering the steady-state concentration of any of these may affect our behavior. The initial working hypothesis was that excess dopamine in the hrain produces schizophrenia, low levels of serotonin and norepinephrine cause depression, and high levels of these two hiogenic amines produce manic depression (alternating periods of extreme excitement and restlessness). This simple view is no longer adequate; however, a clear picture of the neurotransmitterbehavior relationship has not been formed yet (25). Parkinson's disease patients have a deficiency of dopamine in the hrain resulting from the degeneration of certain nerve cells producing it. Dopamine cannot he given to these patients hecause it does not pass the blood-brain harrier. L-dopa can enter the hrain where it is enzymatically converted to dopamine.
take" process, causing a surplus of these amines to accumulate.
Norepinephrine
A molecular-level probe of the chemoreception of norepinephrine, the primary adrenergic transmitter, has been made by studying fluorinated analogs (21). Interestingly, it was observed that the 6-fluoro-norenhineuhrine is an a-recentor agonist with virtually no P-receptor agonist activity, whereas the 2-fluoro isomer is a B-receotor aeonist with little a-acti" vity, and the 5-fluoro isomer is equipotent with NE as an a-receutor aponist and sienificantlv more ootent as a &receptor'agonist. Since the>-, 5-, and 6-fluok derivativks of dopamine are e a.u i.~ o t e nwith t d o ~ a m i n etoward d o ~ a m i n e receptors as well as a-and P-adrenergic receptors, it implies that the effect of F on the catechol ring is only operative when the P~hydroxylgroup is present. It is probable that H-bonding and altered hydrophobicity of the F analogs are involved and an elucidation of this provides a challenge to physical-organic chemists. I t was found recently that many chemically diverse antidepressants hind to the neurotransmitter histamine receptors linked to an adenylate cyclase. Histamine, like dopamine, has two kinds of receptors, one associated with an adenylate cyclase and one not. Thus. some antidenressants hind to both and the norepinephrine here is unclear. Later, it was found that the hrain also has a receptor selective for benzodiazepines (e.g. Valium and Lihrium).
I
CH,CH.CH,NMe, Imiprarnine (a dibenzazepine)
I CHa
Valium (The world's largest selling preswliption drug)
Librium
The greater the binding affinity of a drug to the benzodiazepine receptor, the more chemically potent it is in relieving anxiety (22). Only henzodiazepines hind to these receptors hut recently an elusive endogenous substance, called gamma-compound, may have been found that hinds even more tightly than do the anti-anxiety drugs (23). In some unknown way, the henzodiazepine receptor is related to the
L-dopa works dramatically to relieve Parkinsonian symptoms (muscular rieiditv. tremors. slowness of movements) when first administ&e& but after Several years of use, most patients develop disabling side effects (26).However, with new information on how dopamine acts in the brain, there is hope that effective anti~arkinsoniandrues can he desiened which will not produce the disturbing side effects of ~ - & q a . In a search for analzesic opiates by the radio-tareine -- - technique, it was discovered that the blain makes its own pain killers, three of which are enkephalins, endorphin, and dynorphin. This is to he expected because it is unlikely that specific opiate or henzodiazepine receptors evolved solely to These prr,tclns :rc1 cm rewptms bind rxuxen,,us sulht~mvr.~. 10 11lm.k p i n , w i t h d p ~ r p t t ~ Iwin;: n ;IU rimes ~ ~ Ix,tvnt n ethan . i - t ~ n d ~ ~ r p%(I l ~ i ntlrnv. . mure thiin mtrrphinv, nnd ^or, ti1111 .; m m . ~)uweriul~ I I H I I~.nkcqhdin.Nm ~ m l ydu these p d ) pel)tide., with ,miate-likc actwit\. ronfr
