(Bloomberg News) JPMorgan Chase & Co. capped the size of the largest U.S. exchange-traded note after it grew more than 20 percent this year, boosting the odds that the security's price may become unhinged from its value.

The JPMorgan Alerian MLP Index ETN will be limited to 129 million shares, the New York-based bank announced last week. The note, which tracks partnerships that don't pay federal income taxes and rely on assets such as oil pipelines to generate cash flow, added 22 million shares this year to 116 million before the announcement, an increase of 23 percent, according to data compiled by Bloomberg.

The restriction may lead the security to trade at prices not based on the underlying index, the so-called indicative value, said Ron Rowland, editor of Invest With An Edge, an exchange-traded funds newsletter. In February, Credit Suisse Group AG stopped issuing shares of a volatility-tied note under the ticker TVIX that a month later began diverging from its underlying index to trade at a premium of as much as 89 percent.

"Capping the number of shares converts these products into de facto closed-end funds, which are very different animals," said Rowland, who is also founder of Capital Cities Asset Management Inc. in Austin, Texas. "This will lead to problems like we saw with TVIX."

Jennifer Zuccarelli, a spokeswoman for JPMorgan in New York, declined to comment.

Investors who want to bet on master limited partnerships can choose from at least nine other exchange-traded products. UBS AG and Credit Suisse offer ETNs, and ALPS Advisors Inc. sells the largest exchange-traded fund tied to an MLP index.

Price Gyrations
The TVIX note lost more than 50 percent of its value in two days in March. After record demand made the security difficult to hedge, Zurich-based Credit Suisse stopped creating new shares of the ETN on Feb. 21, and the price later became dislocated from its indicative value. The U.S. Securities and Exchange Commission is probing price gyrations in the note, a person familiar with the matter said in March.

The JPMorgan Alerian ETN is the largest in the U.S. with a market capitalization of $4.21 billion and makes up more than a quarter of the ETN market. It tracks a gauge of 50 publicly traded master limited partnerships and matches dividends from the Alerian MLP Index minus a 0.85 percent management fee, according to a prospectus filed with the SEC. The partnerships typically invest in assets ranging from pipelines to ships transporting commodities.

ETNs are unsecured bank debt backed by their issuer's credit, unlike exchange-traded funds, which hold assets. Banks create and redeem shares of ETNs based on the level of demand for the securities. That demand typically doesn't affect the price since the ETNs track the performance of an index.