UPDATE 1-Apollo posts bumper Q4 profits on stock market rally

* Q4 economic net income up almost a quarter to $490.3 mln

* Performance boosted by appreciation in private equity fund

* Assets under management $248.9 billion at end-December (Adds CEO comment, details on results and earnings beat)

Feb 1 (Reuters) - Alternative asset manager Apollo Global Management LLC said on Thursday its fourth-quarter economic net income rose by almost 25 percent to $490.3 million year-on-year, boosted by a strong appreciation in its private equity business.

Apollo earnings exceeded Wall Street expectations and showed how 2017's stock rally buoyed company valuations and allowed buyout firms to book gains on some of them at top dollar.

Apollo's private equity funds appreciated 9.1 percent in the fourth quarter, helping drive economic net income per share to$1.22, well ahead of analysts' expectations of 66 cents, according to Thomson Reuters I/B/E/S.

Economic net income reflects the mark-to-market valuation of gains or losses in Apollo's portfolio and is a key earnings metric for U.S. private equity firms.

"Our strong results for the fourth quarter capped an exceptional 2017 for Apollo," Apollo Chairman and Chief Executive Leon Black said in a statement.

Fourth-quarter after-tax distributable earnings - the actual cash available for paying dividends - totalled $317.2 million, up from $226.2 million a year ago, as Apollo cashed out on some holdings at high valuations.

New York-based Apollo's assets under management totalled $248.9 billion at the end of December, up from $242 billion three months earlier. (Reporting by Joshua Franklin in New York; Editing by Lisa Von Ahn and Andrew Hay)