NEW YORK, April 4 (Reuters) - GMAC shored up capital at its struggling Residential Capital unit by buying back $1.2 billion of ResCap debt to ease the mortgage lender's debt burden.

GMAC paid a little more than $600 million in the open market for ResCap notes carrying a face value of $1.2 billion, and in exchange received about $873 million of preferred shares from the unit. ResCap has retired the debt, according to a U.S. Securities and Exchange Commission filing on Friday.

The deal effectively reduces ResCap's debt and lowers its annual debt payments, making the company better able to weather expected losses from its subprime mortgages.

GMAC also bought $340 million of ResCap notes, with a market value of about $266 million, which it can exchange for additional ResCap preferred shares by May 31, according to the filing.

Moody's Investors Service said on Friday the move does not impact the ratings of GMAC or ResCap.

ResCap, the second largest independent U.S. mortgage lender, lost $4.35 billion in 2007 as the housing slump led to higher defaults and lower loan volume.

Standard & Poor's said in February that ResCap might require new capital injections from General Motors Corp (GM.N: Quote, Profile, Research) and private equity firm Cerberus Capital Management LP. [CBS.UL].

A consortium led by Cerberus owns 51 percent of GMAC. General Motors owns the remaining 49 percent. (Reporting by Paritosh Bansal and Dan Wilchins; Editing by Andre Grenon)