EN ESTADOS UNIDOS
4.2 Problemas del Sector
Fetal alcohol syndrome arises due to excessive prenatal maternal ethanol ingestion (Clarren & Smith, 1 978; Jones et al., 1 973 ; Lemoine et al., 1 968). The syndrome is characterised by craniofacial defects of the eyes, upper lip and jaw, growth retardation, and central nervous system dysfunction. The world-wide incidence of full F AS is
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estimated to be 1 .9 per 1 000 live births, and for partial FAS (known as fetal alcohol effects or FAE), 3 -5 per 1 000 live births (Luke, 1 990). Among alcoholic women, the estimated incidence rises to 25 per 1 000 (F AS), and 90 per 1 000 (F AE), making F AS the leading cause of mental retardation in the western world (Streissguth et al., 1 986). Symptoms observed in F AS-affected individuals vary widely, both in the severity of the malformation, and in the type of malformation seen. The molecular basis for ethanol induced teratogenicity is unknown. Current ideas as to how ethanol causes F AS are: 1 ) direct teratogenic effects of ethanol or acetaldehyde on the fetus, 2) altered maternal or placental physiology, and/or 3) nutritional alterations caused by ethanol ingestion (DeJonge & Zachman, 1 995). A nutritional compound proposed to be involved in ethanol-induced teratogenicity is vitamin A (DeJonge & Zachman, 1 99 5 ; Deltour et al. , 1 996; Duester, 1 99 1 ; Duester, 1 994; Grummer et aI., 1 993 ; Grummer & Zachman, 1995; Morriss-Kay & S okolova, 1 996; Pullarkat, 1 99 1 ).
The importance of retinoic acid during embryogenesis in basic proces ses such as cell differentiation, cellular rearrangement and pattern formation, has been well documented (see section 1 . 1 .3 ) . Many defects associated with vitamin A deficiency or toxicity are similar to those observed in F AS-affected individuals; for example, congenital heart defects seen in F AS are similar to those found in vitamin A teratogenesis. (DeJ onge &
Zachman, 1 995). From these general observations, the search for a molecular basis linking F AS to alterations in vitamin A homeostasis was initiated. It is known that fetal and adult vitamin A status is affected by ethanol ingestion (Grummer e t al., 1 993;
Grummer & Zachman, 1 990; Lieber, 1991). More specifically, maternal ethanol
ingestion has been identified to decrease fetal liver retinol levels, increase fetal lung and kidney retinol and retinyl palmitate stores, increase fetal brain retinoids, and increase fetal brain CRABP (Grummer et al., 1 993). Further investigation showed no difference in maternal serum retinol levels, an increase in CRBP in fetal brain and whole embryo, a decrease in RAR0 only in whole embryo, and an increase in RARy in fetal brain in ethanol-treated rats compared to control (Grummer & Zachman, 1 995). It has long been known that ethanol inhibits ADH-catalysed retinol oxidation (Julia et al. , 1 986; Leo
et al., 1 987; Van Thie! et al. , 1 974). This is a proposed mechanism for the manifestation ofF AS (Deltour et al., 1 996; Duester, 1 99 1 ; Duester, 1 994). It has recently been shown that high concentrations of ethanol decreased retinoic acid levels in mouse embryos at stages most sensitive to ethanol induced cranial defects. Class IV ADH mRNA was also expressed at the same time (Deltour et al. , 1 996). Although not direct evidence, these
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experiments support the hypothesis of ethanol induced alterations in retinoic acid production, availability, and/or homeostasis being a mechanism for F AS manifestation.
It is possible that AlDH 1 may also play a role in this scenano. The ability of
acetaldehyde to inhibit aldehyde dehydrogenase-catalysed retinal oxidation has not been
investigated. The ability of AlDH 1 to oxidise free and CRBP-bound retinal, as well as
at least the 9-cis geometric isomer indicates a primary role for AlDH 1 in retinoic acid
homeostasis. In addition, AlDH 1 i s the only aldehyde dehydrogenase isoenzyme with
the ability to oxidise retinal that has been found in humans, and it is also able to oxidise
acetaldehyde. Further studies may reveal a role for AlDH 1 in FAS.
1.6 The Aldehyde Dehydr ogenases
The aldehyde dehydrogenases (aldehyde:NAD+ oxidoreductase, EC 1 .2. 1 . 3 ), are a group
of enzymes which catalyse the oxidation of various aliphatic and aromatic aldehydes to
their corresponding acids (Goedde & Agarwal, 1 990; Pietruszko, 1 98 3 ; Sladek et al.,
1 989). These enzymes are considered to play a general role in detoxification.
Specifically, AlDHs have been shown to metabolise acetaldehyde derived from ethanol,
toxic aldehydes from food and lipid peroxidation (Harrington et al., 1 987; Jakoby &
Ziegler, 1 990; Mitchell & Petersen, 1 987; Parrilla et al., 1 974), retinoids, and aldehydes
derived from biogenic amines and neurotransmitters (Ambroziak & Pietruszko, 1 99 1 ;
Ambroziak & Pietruszko, 1 993 ; Yoshida e t al., 1 992; Yoshida et al., 1 993). On the
basis of various properties, the AlDHs can be classified into 3 main classes (LindahJ &
Hempel, 1 99 1 ). These properties include physicochemical and enzymatic properties,
tissue/subcellular distribution, and sequence identities (Yoshida et al., 1 99 1).
Class 1 enzymes are homotetramers, composed of 54 kDa monomers. They are
cytosoIic, with a fairly broad substrate specificity. Included in this class are the major
human liver cytosolic isofonn (hAlDH 1 ), and the homologues from rat (rAlDH 1),
sheep (sAlDH 1 ) (Figure 1 . 8), mouse
(mAHD
2), and others (note: nomenclature formouse aldehyde dehydrogenases differs from that of the other enzymes). Based upon
multiple sequence alignment (Figure 5 . 1 ), the recently isolated retinal-oxidising
dehydrogenases are likely to be classified as class 1 also. Class 2 enzymes are also
homotetrameric with a similar monomer size to the class 1 enzymes. These are found in
the mitochondria, and are thought to be the major isofonn involved in acetaldehyde
B
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A
c
c
This diagram was constructed from coordinates determined by S . Moore (Massey University, NZ) using the program TURBO-FRODO (Cambillau et ai, 1 996). a-Helices
are coloured red, �-strands blue, and loops yellow, while all-trans retinal is shown in green. All 4 subunits are shown (labelled A-D) each containing a retinal molecule.
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isoform (GIu to Lys at position 487) is involved in the 'flushing syndrome', or adverse
reaction to alcohol consumption suffered by a large percentage (�50 %) of Asian people
(Harada et aI. , 1 98 1 ; Ikawa et al. , 1 983). The class 3 enzymes are dim eric, with a
subunit molecular weight of 50 kDa, and include corneal AlDH and a tumour-associated
AlDH.
The hypothesis for the general mechanism of AlDH 1 catalysed NAD+ -dependent
oxidation of aldehydes is supported by studying the recently solved structure of this enzyme (S. Moore, personal communication) (Figure 1 . 8) . The proposed kinetic mechanism is nucleophilic attack by Cys 3 02 on the carbonyl of the aldehyde substrate to form an acyl-intermediate, followed by hydride transfer to Glu 399. Deacylation then
occurs, producing the acid, NADH and regenerated enzyme ( Sheikh et aI. , 1 997).