and the enantiomers have markedly different potencies. In the cases of both 4-en-VPA and 4-yn-VPA, the S-enantiomer is more potent than the racemate and the R-enantiomer is virtually inactive (Andrews et al. 1997, 1995; Hauck and Nau 1992).

These structure-activity relationships are not due to pharmacokinetic differences, as shown by direct measurements of tissue levels and by the activities of VPA and analogs in embryo culture (Brown et al. 1987; Nau 1994; Andrews et al. 1997, 1995). They are also consistent across species (Andrews et al. 1997, 1995). The overall impression is that the teratogenic effect of valproids requires an interaction with a specific site, at which one alkyl chain becomes located in a hydrophobic pocket, thus enabling ionic bonding of the carboxyl group and the interaction of the second chain with a region that favors the high electron density of terminal unsaturation (Bojic et al. 1998).

Chemicals That Might Induce Apoptosis

More than 1,000 agents have been identified as teratogens in animal studies (Shepard 1998); moreover, a variety of studies has now shown that cell death is an early, common event in the teratogenic process initiated by many, if not all, teratogens (Scott 1977; Knudsen 1997). Often, teratogen-induced cell death occurs preferentially in areas of normal programmed cell death, suggesting that there might be a mechanistic link between programmed and teratogen-induced cell death (Alles and Sulik 1989). The importance of an appropriate amount of programmed cell death to normal development is highlighted by mouse mutants, such as Hammertoe, in which insufficient programmed cell death underlies abnormal limb development (Zakeri et al. 1994). Likewise, excessive teratogen-induced cell death is directly linked to abnormal development by the finding that 2-chloro-2′-deoxyadenosine-induced eye defects are associated with excessive teratogen-induced cell death (Wubah et al. 1996).

Recent research has shown that cell death induced by a variety of stimuli occurs by a process termed apoptosis. Although many of the details are still lacking, it is known that apoptosis is a tightly controlled process, triggered either internally or externally, by which a cell self-destructs in a manner that does not lead to destruction of neighboring cells. Key components in the execution phase of the apoptotic pathway are the intracellular cysteinyl-aspartate proteases known as caspases, particularly caspase-3 (Colussi and Kumar 1999). These enzymes are normally present in all cells as inactive precursors that become activated by cleavage at specific internal motifs. Once activated, these caspases function to degrade specific target substrates, such as poly(ADP-ribose)polymerase (PARP), DNA-PKs, and lamins. A recent report shows that developmental toxicants, such as hyperthermia, cyclophosphamide (an alkylating agent), and sodium arsenite (a thiol oxidant), induce increased cell death characterized by activation of caspase-3, cleavage of PARP, and fragmentation of DNA (Mirkes and Little 1998). Although some of the downstream events in the apoptotic pathway activated by

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