Abstract
Silkworms contain high concentrations of D-serine, an optical isomer of L-serine. Serine racemase catalyzes conversion of free L-serine to D-serine and vice versa. To elucidate the physiological role of the D-amino acid, we examined effects of O-phospho-L-serine (OPLS) on silkworm growth and development by adding it to the silkworms’ diet because we had found that OPLS competitively inhibits serine racemase prepared from the pupae. The result clarified OPLS effects: larval development was delayed by approximately 6 days, and the larval survival rate was significantly decreased. These effects were reversed toward the control levels by D-serine administered together with OPLS. Body D-serine level was lower in the OPLS-administered larvae than in the control ones right up until the spinning stage. D-Serine levels in D-serine- and L-serine-administered larvae were markedly higher than in the control larvae, but the growth rate and the survival rate were lower than those of the control group. The L-serine may be converted to D-serine by the catalytic function of serine racemase in the body. The D-serine concentration in feces excreted from these larvae was markedly high suggesting contribution of feces in maintaining a proper body D-serine level. All these results together suggest that D-serine is necessary for normal growth and development of the silkworm, while excess amino acid reversed its function.
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