Bombyx mori can synthesize ascorbic acid through the l‐gulose pathway to varying degrees depending on developmental stage

Abstract

Vitamin C (VC) is an essential nutrient for many animals. However, whether insects, including Bombyx mori, can synthesize VC remains unclear. In this article, the optimized HPLC method was used to determine the content of l‐ascorbic acid (AsA) in silkworm eggs, larvae and pupae, and the activity of l‐gulono‐1,4‐lactone oxidase (GULO), a key enzyme in VC synthesis. The RNA interference method was used to determine the effect of the BmGulo‐like gene on embryonic development and GULO activity in the pupal fat body. The AsA content increased significantly during E144 h–E168 h in the late embryonic stage and P48 h–P144 h in the middle‐late pupal stage, in which exogenous VC was not ingested. Furthermore, the body AsA content in larvae fed VC‐free feed also increased with larval stage. The GULO enzymatic activity was present in eggs and the fat bodies of larvae and pupae, even when the larvae were reared with fresh mulberry leaves. Moreover, the activity was higher in the later embryonic stages (E144 h–E168 h) and the early pupal stage (before P24 h). The GULO activity in the pupal fat body dramatically decreased when the screened BmGulo‐like gene (BGIBMGA005735) was knocked down with small interfering RNA; in addition, the survival rate and hatching rate of eggs significantly decreased 21% and 44%, respectively, and embryonic development was delayed. Thus, Bombyx mori can synthesize AsA through the l‐gulose pathway, albeit with low activity, and this synthesis ability varies with developmental stages.

The evolution of vitamin C biosynthesis and transport in animals

“…Since only one VC synthesis pathway has been described in animals, species belonging to these lineages were thought not to be able to synthesize VC. Nevertheless, at present, there are two insect species, namely Drosophila melanogaster (diptera), and Bombyx mori (lepidoptera), as well as one nematode ( Caenorhabditis elegans ), that have been shown to synthesize VC [ 18 – 20 ]. It should be noted that distantly related insect groups, such as the Diptera and the Lepidoptera, may have independently acquired de novo the ability to synthesize VC.…”

“…In B. mori , it has been suggested that the VC synthesis pathway could be identical to that of mammals and other vertebrates with the exception of the enzyme involved in the last step of the pathway [ 20 ]. Indeed, GULO-like activity has been detected in B. mori , although at reduced levels compared to other species [ 20 ].…”

“…In B. mori , it has been suggested that the VC synthesis pathway could be identical to that of mammals and other vertebrates with the exception of the enzyme involved in the last step of the pathway [ 20 ]. Indeed, GULO-like activity has been detected in B. mori , although at reduced levels compared to other species [ 20 ]. Surprisingly, the protein identified as having GULO-like activity is a sterol reductase, in contrast to GULO that is an aldonolactone oxireductase [ 20 , 22 ].…”

“…Indeed, GULO-like activity has been detected in B. mori , although at reduced levels compared to other species [ 20 ]. Surprisingly, the protein identified as having GULO-like activity is a sterol reductase, in contrast to GULO that is an aldonolactone oxireductase [ 20 , 22 ]. Since aldonolactone oxireductases and sterol reductases are distantly related flavoproteins [ 22 ], the ability of the B. mori sterol reductase to catalyze the oxidation of l -gulonolactone was unexpected.…”

“…The former hypothesis is highly unlikely. For instance, although the lepidopteran B. mori is able to synthesize VC, it does not synthesize enough of it during larval stages 3-LE up to 5-LE for its needs, and thus must rely on an external source of this nutrient [ 20 ]. Therefore, B. mori must have a functional VC transporter.…”