Juvenile Hormone

An important hormone in insects is the so-called juvenile hormone, produced in the corpora allata, a part of the brain. It acquired that name because it is required at each moult between immature stages (see Figure 1.4). When JH is absent, the insect moults to the adult form. But the hormone is also produced in adult insects and has other functions there. Pheromone release in Lepidoptera and Coleoptera is often under the control of JH.

The juvenile hormone first discovered, known as JH I, is a bishomo-terpene (Figure 6.27). Later, all the forms from JH I to JH III, corresponding to the farnesene homologues shown above have been found, but only JH III is found in all insect orders. The hormones JH 0 to JH III are found only in Lepidoptera. Notice that a different isomer, 3-methyl-3-pentenyl pyrophosphate is added in 4-methyl-JH I compared to the others (Figure 6.27). Similar homofarnesene skeletons are also found in ants. JHB3, an epoxide form of JH III, has been found in all the higher flies (Diptera) so far studied. Biosynthetic studies have firmly shown that JH III is produced via mevalonate or homomevalonate, as required, and then through the steps already discussed to farnesyl pyrophosphate or homofarnesyl pyrophosphate. The action of pyrophosphatase releases free farnesol, which is oxidized by a dehydrogenase and NAD+ to

h jhb3

4-methyl-JH I

Figure 6.27 The known forms of juvenile hormone found in insects. The structure of 4-methyl-JH I is explained in Figure 6.22

h jhb3

4-methyl-JH I

Figure 6.27 The known forms of juvenile hormone found in insects. The structure of 4-methyl-JH I is explained in Figure 6.22

farnesal and then to farnesoic acid. This is esterified using S-adenosyl methionine and a methyl transferase. Finally methyl farnesoate is epoxi-dized by a cytochrome P450 to juvenile hormone. These stages have been thoroughly investigated, for example in the yellow fever mosquito Aedes aegypti.

It is interesting that methyl farnesoate is present in some Crustacea, and a mixture of methyl farnesoate and JH III in the embryos of primitive insects such as cockroaches. There appears to be a kind of evolution through appearance of successive epoxidases of the cytochrome P450 family. Some form of juvenile hormone is present in ticks and other arthropods.

Juvenile hormone is inactivated by hydrolysis of the methyl ester followed by opening of the epoxide ring to a diol. The epoxide hydrase works 17 times faster on the JH III acid than on the JH III itself.

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