Identification of the substrate recognition region in the Δ⁶-fatty acid and Δ⁸-sphingolipid desaturase by fusion mutagenesis

Δ⁸-sphingolipid desaturase and Δ⁶-fatty acid desaturase share high protein sequence identity. Thus, it has been hypothesized that Δ⁶-fatty acid desaturase is derived from Δ⁸-sphingolipid desaturase; however, there is no direct proof. The substrate recognition regions of Δ⁶-fatty acid desaturase and Δ⁸-sphingolipid desaturase, which aid in understanding the evolution of these two enzymes, have not been reported. A blackcurrant Δ⁶-fatty acid desaturase and a Δ⁸-sphingolipid desaturase gene, RnD6C and RnD8A, respectively, share more than 80 % identity in their coding protein sequences. In this study, a set of fusion genes of RnD6C and RnD8A were constructed and expressed in yeast. The Δ⁶- and Δ⁸-desaturase activities of the fusion proteins were characterized. Our results indicated that (1) the exchange of the C-terminal 172 amino acid residues can lead to a significant decrease in both desaturase activities; (2) amino acid residues 114-174, 206-257, and 258-276 played important roles in Δ⁶-substrate recognition, and the last two regions were crucial for Δ⁸-substrate recognition; and (3) amino acid residues 114-276 of Δ⁶-fatty acid desaturase contained the substrate recognition site(s) responsible for discrimination between ceramide (a substrate of Δ⁸-sphingolipid desaturase) and acyl-PC (a substrate of Δ⁶-fatty acid desaturase). Substituting the amino acid residues 114-276 of RnD8A with those of RnD6C resulted in a gain of Δ⁶-desaturase activity in the fusion protein but a loss in Δ⁸-sphingolipid desaturase activity. In conclusion, several regions important for the substrate recognition of Δ⁸-sphingolipid desaturase and Δ⁶-fatty acid desaturase were identified, which provide clues in understanding the relationship between the structure and function in desaturases.

Identification and functional analysis of the genes encoding Delta6-desaturase from Ribes nigrum

Gamma-linolenic acid (gamma-linolenic acid, GLA; C18:3 Delta(6, 9, 12)) belongs to the omega-6 family and exists primarily in several plant oils, such as evening primrose oil, blackcurrant oil, and borage oil. Delta(6)-desaturase is a key enzyme involved in the synthesis of GLA. There have been no previous reports on the genes encoding Delta(6)-desaturase in blackcurrant (Ribes nigrum L.). In this research, five nearly identical copies of Delta(6)-desaturase gene-like sequences, named RnD8A, RnD8B, RnD6C, RnD6D, and RnD6E, were isolated from blackcurrant. Heterologous expression in Saccharomyces cerevisiae and/or Arabidopsis thaliana confirmed that RnD6C/D/E were Delta(6)-desaturases that could use both alpha-linolenic acids (ALA; C18:3 Delta(9,12,15)) and linoleic acid (LA; C18:2 Delta(9,12)) precursors in vivo, whereas RnD8A/B were Delta(8)-sphingolipid desaturases. Expression of GFP tagged with RnD6C/D/E showed that blackcurrant Delta(6)-desaturases were located in the mitochondrion (MIT) in yeast and the endoplasmic reticulum (ER) in tobacco. GC-MS results showed that blackcurrant accumulated GLA and octadecatetraenoic acids (OTA; C18:4 Delta(6,9,12,15)) mainly in seeds and a little in other organs and tissues. RT-PCR results showed that RnD6C and RnD6E were expressed in all the tissues at a low level, whereas RnD6D was expressed at a high level only in seeds, leading to the accumulation of GLA and OTA in seeds. This research provides new insights to our understanding of GLA synthesis and accumulation in plants and the evolutionary relationship of this class of desaturases, and new clues as to the amino acid determinants which define precise enzyme activity.

Use of cold iodine in patients with Graves' disease: observations from a clinical practice

OBJECTIVE

To determine the efficacy and safety of cold iodine after radioactive iodine therapy (RAI) and the effects of prior antithyroid drug therapy in patients with Graves' disease.

METHODS

We undertook a review of medical records of 280 consecutive patients with Graves' disease who were treated with RAI followed by cold iodine. Logistic regression methods were used to model treatment failure.

RESULTS

Of the 280 patients, 246 (88%) were successfully treated with a single dose of RAI, whereas 34 patients (12%) required further therapy. The mean duration of time to achieve euthyroidism or hypothyroidism in successfully treated patients was 68 days. Of the 209 patients who were never given antithyroid drugs (ATDs), 190 (91%) were successfully treated with a single dose of RAI. No differences were found in patient age, thyroid gland size, or initial free thyroxine index between the successfully treated group and the treatment failure group among those who received RAI alone. Of the 71 patients who had been treated with ATDs before RAI administration, 56 (79%) were successfully treated with a single dose of RAI. The treatment failure rate of 21% in patients who had received ATDs was significantly higher than the 9% failure rate observed in those who were never given ATDs (P = 0.01). Multivariate analysis showed that larger gland size increased the likelihood of treatment failure [c2(2) = 11.76)] and prior treatment with ATDs doubled the risk of treatment failure after adjusting for gland size. No serious adverse events were noted after RAI or cold iodine therapy.

CONCLUSION

Treatment of Graves' disease with RAI followed by cold iodine was safe and effective. The use of ATDs before 131I ablation resulted in a 2.3-fold higher treatment failure rate. ATDs need be used only in high-risk persons, and if such therapy is undertaken, higher doses of 131I should be administered to reduce the incidence of treatment failure.