* BOJ buyback operations fail to provide much support * Investors eye Japan life insurer investment plans By Lisa Twaronite TOKYO, April 24 (Reuters) - Japanese government bonds slipped on Wednesday as surging stocks stoked risk sentiment, with the yield curve steepening as the superlong tenor underperformed. The Bank of Japan's latest round of purchases under its massive new stimulus scheme failed to provide much support. The BOJ offered to buy 500 billion yen ($5.03 billion) in JGBs outright, with residual maturities of more than 5 years and up to 10 years, and another 500 billion yen with residual maturities of more than 1 year and up to 5 years. "The operation expectations drive the market on both sides, and although volatility has come down quite a bit, especially in the medium-term sector, still some concerns remain for the superlong-term tenor," said Maki Shimizu, senior strategist at Citigroup in Tokyo. "The BOJ will eventually absorb up to the 10-year sector, to a significant degree, but what's a concern is which of the superlong tenors will be bought back, because the operation scheme is still not very detailed," she said. The yield on benchmark 10-year bonds rose 1 basis point to 0.595 percent. The 10-year futures contract edged down 0.06 point to 144.61, losing its grip on earlier gains as the Nikkei share average jumped 2.3 percent to its highest level in nearly five years. Market participants continued to scan the investment plans of Japanese life insurance companies for any signs they might shift their allocations to foreign bonds from JGBs as the BOJ doubles its bond holdings over two years under its easing plan. On Wednesday, an executive at Meiji Yasuda Life Insurance Co, Japan's third-largest private life insurer, said it is taking a cautious stance on JGBs, and that it plans to buy around 500 billion yen of foreign bonds in the fiscal year that began this month. An official at Asahi Mutual Life Insurance said it will also be more cautious about buying Japanese bonds this fiscal year and will seek opportunities to buy foreign bonds on dips. The BOJ will update its forecasts after its policy meeting on Friday. While no major policy steps are expected at the meeting, investors will be watching for any operational tweaks to the central bank's asset-buying programme, and also whether BOJ Governor Haruhiko Kuroda's two-year time frame to hit a 2 percent inflation target will become the bank's official forecast. Analysts say such a prediction might be too optimistic, and could put the bank's credibility on the line. Most analysts polled by Reuters after the BOJ's policy meeting on April 4 said the BOJ will not be able to achieve its inflation target within two years. The superlong sector underperformed, as the BOJ refrained from any buying in that tenor this time. The 20-year yield added 3.5 basis points to 1.480 percent, while the 30-year yield rose 2.5 basis points to 1.600 percent. "The JGB market is making relatively small moves, on supply/demand factors as investors adjust their portfolios, and these conditions could continue through the Golden Week holidays," said a fixed income fund manager at a European asset management firm in Tokyo. Tokyo markets will be closed for holidays on April 29, and then on May 3 and May 6.