* Apple biggest drag on S&P, Nasdaq
* Exxon, Chevron fall as crude prices slide
* Valeant slumps as CEO goes on medical leave
* Disney up as latest Star Wars rakes in $1 bln
* Indexes down: Dow 0.46 pct, S&P 0.56 pct, Nasdaq 0.67 pct (Updates to early afternoon)
By Abhiram Nandakumar
Dec 28 (Reuters) - Wall Street began the last trading week of the year in negative turf, as the oversupply pressure on crude oil prices showed no signs of abating.
The S&P 500 energy sector was down 1.87 percent, easily the worst performer on the index. Exxon's shares were down 1 percent at $78.57, while Chevron was down 1.9 percent at $90.34.
The S&P 500 slipped back into negative territory for the year on Monday, down 0.5 percent, while the Dow Jones industrial average was down 2 percent. The Nasdaq Composite index was up 5.8 percent.
"It's mostly on crude. That's more the issue than anything else," said Paul Nolte, senior vice president and portfolio manager at Kingsview Asset Management in Chicago.
U.S. stock indexes have closely tracked crude prices in the past several weeks.
The three major indexes are coming off their best week since mid-November, having risen about 2.5 percent last week, led by a short-lived surge in energy stocks.
At 12:32 p.m. ET (1732 GMT), the Dow Jones industrial average was down 80.55 points, or 0.46 percent, at 17,471.62, the S&P 500 was down 11.55 points, or 0.56 percent, at 2,049.44 and the Nasdaq Composite index was down 33.89 points, or 0.67 percent, at 5,014.60.
Apple was down 1 percent at $106.97 and was the biggest drag on the S&P and Nasdaq. Goldman Sachs' 1 percent decline weighed the most on the Dow.
Trading volumes are expected be subdued through the week, which is likely to exacerbate volatility.
Valeant was down 10 percent at $102.66 after the Canadian drugmaker said Chief Executive Michael Pearson is going on medical leave.
Fitbit was up 4 percent at $30.07, after reports that the wearable gadget maker's iOS app was the most downloaded after Christmas, suggesting strong holiday demand.
Dow component Disney was up 0.9 percent at $106.86, after the company's latest Star Wars installment topped $1 billion in ticket sales.
Declining issues outnumbered advancing ones on the NYSE by 2,218 to 761. On the Nasdaq, 1,920 issues fell and 824 rose.
The S&P 500 index showed no new 52-week highs and no new lows, while the Nasdaq recorded 28 new highs and 35 new lows. (Reporting by Abhiram Nandakumar in Bengaluru; Editing by Don Sebastian)